Although these days you don’t really need to be all that good at doing math in your head, it can still be a useful skill. Today we’re going to look at certain patterns and ways of thinking that can help you store, calculate, and work with numbers both in your head and on paper. While some of these tips are more general, others will deal with very specific situations or even real life scenarios like calculating tips. These are 25 simple mental math tricks that will blow your mind!
Divisibility by 3
To see if a number is divisible by three, add up the digits. If the sum is divisible by three, so is the number. Note: this also works for 9.
x% of y is the same as y% of x. This means that if 2% of 50 is too hard to figure out, you can just flip it – 50% of 2. That’s a lot easier.
Multiplying by 11
To multiply a 2 digit number by 11 just add the digits together and stick the answer between them. For example 42*11 would be 4 + 2 = 6. Now put the 6 between the 4 and the 2 to get 462.
Note: Don’t forget to carry the 1. To give another example, 58*11 would be 5 + 8 = 13. You can’t just put the 13 between the 5 and the 8 though. That would give 5138, which is obviously wrong. You need to carry the one by adding it to the 5. So it would be 638.
Multiplying by 5
If you need to multiply a number by 5 it may be easier to half it and then multiply by 10. For example, 315*5 would be 315 / 2 = 157.5 Now just move the decimal over to multiply by 10 and get 1575.
Approximating a square root
To approximate a square root, do the following. Find the next lower perfect square. Add it to your number. Now divide by the square root of the lower perfect square. And now divide again by 2. Sounds hard? Here’s an example. Take 31. The next lowest perfect square is 25. So 31 plus 25 is 56. Now divide by the square root of 25. So 56 / 5 = 11.2 Now divide by 2 to get 5.6
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