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February 16 2008
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10 Easy Math Tricks You Probably Don't Know

math, problem solving, equationMany people are terrified of math, but even if you let out a great big sigh of relief when you got out of school (so you wouldn’t have to think about any more arithmetic), you probably soon realized that math was still necessary in the “real world.”

Whether you love math, or despise it, the following tricks can turn you into a math whiz -- or at least will help to speed up some of the calculations you need to do in your head.

I’ve included five of my favorites here (the link below has the entire list).

1. Quick Square

If you need to square a 2-digit number ending in 5, multiply the first digit by itself plus 1, and put 25 on the end. That’s all!

252 = (2x(2+1)) & 25

2 x 3 = 6

625

2. Multiplying by 9

To multiply any number between 1 and 9 by 9, hold both hands in front of your face, with fingers extended. Now drop the finger that corresponds to the number you are multiplying (for example, for 9 x 3, drop your third finger). Now count the fingers before the dropped finger (in the case of 9 x 3 it is 2) -- that‘s your first digit. Then, count the fingers after (again in this case, it’s 7). The answer is 27.

3. Dividing by 5

To divide a large number by five, all you need to do is multiply by 2 and move the decimal point one space to the left:

195 / 5

Step 1: 195 x 2 = 390
Step 2: Move the decimal left; 39.0, or just 39

2978 / 5

Step 1: 2978 x 2 = 5956
Step 2: 595.6

4. Subtracting from 1,000

To subtract a large number from 1,000, subtract all but the last number from 9, then subtract the last number from 10:

1000 – 648 = ?

Step 1: subtract 6 from 9 = 3
Step 2: subtract 4 from 9 = 5
Step 3: subtract 8 from 10 = 2

Answer: 352

5. Calculate a Tip

If you need to leave a 15% tip, here is the easy way to do it. Work out 10% (divide the number by 10), then add that number to half its value and you have your answer:

15% of $25 = (10% of 25) + ((10% of 25) / 2)

$2.50 + $1.25 = $3.75
Sources:

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Community Comments ( 40 )
Comment on this Article
  
  
foxtroter_203
[ Joined on 09/06 ] [ Posted on January 30, 2008 ]
16 Points        
   
 
Savvy User
What is the mathematical trick for solving the following:

(FDA + FBI) ÷ (CIA + IRS) = ????????
 [ Reply ]
Mercola
  
Russ Bianchi
[ Joined on 09/06 ]  [ Posted on January 30, 2008]
11 Points        
   
Savvy User
  Mercola
The Sum Of The Equation Is: Weapons of Mass Destruction Divided By K Street Lobbyists To The Negative Multiple Trillions Of Taxpayer Dollars...  Or In Mathmatical Shorthand = "A Black Hole"...
Mercola
  
New to Natural
[ Joined on 11/07 ]  [ Posted on January 31, 2008]
17 Points        
   
Savvy User
  Mercola
I was thinking "B.S. squared" myself.
Mercola
  
miragemama
[ Joined on 06/07 ]  [ Posted on January 31, 2008]
17 Points        
   
Apprentice User
  Mercola
= Screwed


Mercola
  
kcrist
[ Joined on 06/06 ]  [ Posted on February 16, 2008]
       
   
Novice User
  Mercola

screwed

Mercola
  
alm260
[ Joined on 06/06 ]  [ Posted on February 16, 2008]
       
   
Savvy User
  Mercola

= slavery

  
  
s-orion
[ Joined on 03/07 ] [ Posted on February 16, 2008 ]
7 Points        
   
 
Novice User

There are many ivory towers for mathematicians, but applied mathematicians have a basic need to serve others, by helping solve real world problems. They are condemned to wander the world, masquerading as engineers. There are few, but that is good, because few are needed. They carry calculus, differential equations, matrices, and linear transformations like tools in their bag. They may share the explanation of tricks, but never pull them on others, because they know, it is the height of arrogance and abuse of talent to laud over and not serve others, less understanding than they.

Please do not be too hard on the ivory tower mathematicians; many of their flights of fancy-castles in the air have found practical application. There need to be both kinds.

A trick for you all that has practical application; when the balance in your checkbook doesn’t square with the bank’s number, it is a good common thing to look for the exact difference somewhere in your check register or on the bank sheet. That’s OK, and if you find it, that’s great.

But if not, before doing more, divide that difference, by nine. If it divides evenly, there is a good chance you have transposed two adjacent numbers somewhere; an easy error to, exasperatingly, overlook. No guarantees, just a possible clue to minimize the tearing out of hair!

Good Luck and Steady Winds

 [ Reply ]
  
  
DizzyIzzy1
[ Joined on 06/07 ] [ Posted on January 31, 2008 ]
7 Points        
   
 
Savvy User
This sounds like an overly complicated form of Vedic Maths.

Vedic Maths is literally the only way I passed anything at all at school - maths is NOT my strong point. Thank goodness I came across this in time!! Basically it uses little number tricks and easy-to-remember word rhymes (14 of them) to solve maths problems in a far more cohesive, natural, intuitive way than the standard system. You can literally multiply or divide in the tens or hundreds of thousands in your head in seconds. Dunno why it isn't more widely taught; you could be done with maths lessons by the time you're 10.

http://www.vedicmaths.org have a good introduction for those interested.
 [ Reply ]
  
  
Aaltrude
[ Joined on 04/07 ] [ Posted on January 30, 2008 ]
6 Points        
   
 
Moderator User
I prefer my method for dividing by 5.

Divide by 10 and double the answer.

eg. 20 divided by 5
       20 divided by 10 equals 2, times 2 equals 4

I always loved maths.
 [ Reply ]
  
  
Pro Water Purity
[ Joined on 02/07 ] [ Posted on February 15, 2008 ]
4 Points        
   
 
Novice User

Trick for multipying by 11

Take the number you are multipllying 11 by......say 23 X 11. Take the first numerial of that number, 2, and place it to the right side of the answer, then take the second numerial, 3, and place it to the left side

of the answer, now add the two numerals together 2 + 3, which equals 5 and place it in the middle of the answer.....2 5 3. If the number is 29 X11, you still put the 2 to the left, the 9 to the right, add 2 + 9 which equals 11, so put the 1 in the middle and then add 2+1, so your answer is 319.

Another trick for multiplying with nines : Every answer equals 9

Example: 1 X 9 = 09  (0+9=9)

              2 X 9 =18 (1+8=9)

              3 X 9 = 27(2 +7=9)

              4 X 9 = 36(3+6=9)

              etc.

Also when you multiply in progression, the answer of the first numeral starts at zero, one, two, three,etc and the second numeral starts at 9 ,then 8, then 7 (look at the answers above to see numerical progression)

Here is a rhyme I taught when I was teaching multiplication and division of fractions that made it easy to remember what to do:

Multiplying fractions, no big problem

top times the top

and bottom times the bottom,

Dividing fractions, easy as pie

Flip the second number and multiply

EX: 1/2 divided by 1/4 = 1/2 X 4/1 = 4/2 =2

I always loved math. I thought it was a fun and wonderful game. My greatest achievement was teaching high level math as a volunteer at my son's elementary school. I set out to change an attitude about math being horrible, and I knew it had worked when more than one of the students (now in high school) I taught, would say aren't you that Math Superstar lady? Then they'd sing one of my math jingles or recite one of our tricks. By the end of the program, we had 98% of the students participating weekly in an extracurricular upper level math program. Let's get teachers who love math into the elementary level math programs to build solid basics for students!

 [ Reply ]
  
  
s-orion
[ Joined on 03/07 ] [ Posted on February 16, 2008 ]
3 Points        
   
 
Novice User

Whitebuffalobk and and Sonnie Moonie are so right. When in a small college, I took all the math courses I could, from the engineering arm, not the sciences arm of the school, thus, unwittingly, creating an applied mathematics major for myself, where there was no formal one.

Had to go to the big university at the capitol to take my Graduate Record Examination (GRE). Consisted of a general part and then a second part in one’s major. Before each part, multiple choice answer sheets were handed out, that were to be marked. All students, (about 400 in the auditorium), got an idea of the difficulty of the questions to come, by the number of answers to be marked on the sheets. They were abuzz as the history and English majors had 300 +, the chemistry and physics majors nearly 200. Finally, the call came for math majors to come down and get their sheet. Only one other person got up and came down with me to each get our sheet. We gasped, as there were only 80 or so answer places.

I carefully noted where he sat back down way over on the other side and in front of me in the auditorium. I did my best, but when I got to the last five questions, I had never seen them before. However, I had been well grounded in applying the principles and slowly worked my way through them all, getting done with a modest time left before the buzzer. The other student worked right up to the end, before he turned in the question pamphlet and answer sheet.

I eagerly waited for him to come out to the foyer to talk about the test. Turns out, he was from the big University. To my surprise, he said he was pretty sure he had seen the tricks to most of those last ten tough questions, but couldn’t remember the tricks exactly. He struggled to guess as best he could, but ran out of time and didn’t even get to mark anything down on some of them.

It was the first of many clues I was to get in life, about tricks versus basics and applied mathematicians versus mathematicians.

(help in next email)

 [ Reply ]
  
  
collossus999
[ Joined on 11/06 ] [ Posted on February 15, 2008 ]
3 Points        
   
 
Novice User

Math is full of patterns. If you look out for them, it makes it all a lot simpler.

 [ Reply ]
  
  
Russ Bianchi
[ Joined on 09/06 ] [ Posted on January 30, 2008 ]
3 Points        
   
 
Savvy User
The square of ZERO is ZERO, just like enforcement from FDA on harmful drugs until there is a body count.
 [ Reply ]
  
  
tomjot
[ Joined on 12/06 ] [ Posted on January 31, 2008 ]
2 Points        
   
 
Novice User
  Question: At any given rate of compounding interest, how soon will your investment double?

Answer: Divide the rate into 72!  Magic!

Thusly: 72 divided by 15(%) = 4.8 years to double.  72 divided by 6(%) = 12 years.     72 divided by 9.2(%) = 7.82 years.

Tom
 [ Reply ]
  
  
New to Natural
[ Joined on 11/07 ] [ Posted on January 31, 2008 ]
2 Points        
   
 
Savvy User
I'm going to print this and share with my study hall this afternoon !  You have to love the symmetry and patterns in math.  In a world of chaos, sometimes it's nice to have absolutes! 
 [ Reply ]
  
  
whitebuffalobk
[ Joined on 08/07 ] [ Posted on February 16, 2008 ]
1 Points        
   
 
Novice User

As a math teacher myself...both high school and college level...are you going to try to memorize a trick to do every type of arithmetic problem?  Why not just learn how to do the problems correctly to start with.  Tricks and jingles which work with just a few problems may be "fun" but they do not create concept skills that build upon one another in high school math.  And when these students get into one of the upper-level math classes which expect this background to be in place, not only do they have an answer as to why a "trick" works, they often resent the teacher who is trying to get them to really learn a concept.  Math consists of more than just arithmetic and tricks.  It's an exciting, beautiful subject in its own accord.  Please treat it with the respect it deserves and stop trying to side-track students who could actually learn the subject matter with tricks.  Maybe then the failure level in freshman college level classes will finally go down.

 [ Reply ]
Mercola
  
Sonny Moonie
[ Joined on 12/07 ]  [ Posted on February 16, 2008]
4 Points        
   
Novice User
  Mercola

This article and the comments are about adults keeping their arithmetic skills fresh and using shortcuts because they recognize why they work, not memorizing "jingles" that give answers without knowing why they work.

The funny thing is, the way you want to teach arithmetic is to make students stick to just memorizing the addition and multiplication tables and one set of standard routines that work for all arithmetic problems and having no exposure to anything else at all. Then students have no sense of how to double check anything, ability to recognize reasonable and efficient methods, or experience with using different methods and seeing that they get the same results. So they have no concept of such a thing as methods that can be abstractly defined and proven to work, no experience with finding patterns and finding proofs that they work, and no sense of the use of anything in mathematics except getting the right number for a test answer.

Higher mathematics is mostly games and tricks, along with abstract proofs that they work, as in hunter3738's comment, and to be appreciated as an art form, more than used for anything. You're the one whose advice would keep students stuck at a rote memorization level rather than understanding more mathematics.

Mercola
  
dbs
[ Joined on 02/08 ]  [ Posted on February 16, 2008]
1 Points        
   
Novice User
  Mercola

From one math teacter to another, I agree with you 100%  While students are concentrating on learning tricks, they could be learning the concepts.  Let them discover the real "trick" to a high ACT score or a college placement exam that allows placement above the remedial level and then we've done something!

Mercola
  
Pat Ormsby
[ Joined on 06/06 ]  [ Posted on February 16, 2008]
7 Points