RT lifehacker: Get better at mental math withe these handy shortcuts
Mental subtraction is easiest when you can subtract each digit without having to borrow from the next place up. If the second number has some bigger digits than the first, it gets more complicated. To avoid borrowing places, you want to get rid of those bigger digits.
Here’s how: Say you’re calculating 925-734. That tens place makes things a little complicated. It’d be easier to calculate 925-7AdvertisementAll multiples of 3 have digits that add up to 3 . Multiples of 4: Ignore everything from the hundreds place up. Divide the remaining two-digit number in half. Then run the multiples-of-2 test.Multiples of 7: There areDouble the units and subtract from the tens. E.g., 1365→136−=126→12−=0. If the chain ends in 0 or a multiple of 7, then the original number is divisible by 7.Multiples of 8: Ignore everything from the thousands place up. Divide the remaining three-digit number in half. Then in half again. Then run the multiples-of-2 test.To test divisibility by a larger number, try to factor it down to single-digit numbers, then run the tests above, keeping any repeated factors together. For example, 60=2*2*3*5. So all multiples of 60 are also multiples of 2*2, 3, and 5. Note the 2*2; a multiple of 60 must be divisible by 4, not just by 2. To multiply in your head, try turning the problem into an easier one. For example: Doubling numbers tends to be easier. So when multiplying by an even number, first multiply by half that number, then by 2.Multiply by 9: Multiply by 10 and subtract the number. So 65*9=-65=650-65=585.-1. The second digit is 9 minus the first digit. So 8*9=72.The more basic calculations you’ve memorized, the more you can break down bigger math problems. If you’ve forgotten your times tables,Find a square number slightly bigger than the biggest one you know If you know the square of a whole number, you can easily find the square of the next whole number, by adding the first square, the first root number, and the second root number:
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