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10 Easy Math Tricks You Probably Don't Know

February 16, 2008 | 189,852 views

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 and References

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