You probably haven’t had to do handwriting math in years, but you do mental math every day. Or maybe you Google math problems ten times a day because you’ve forgotten how to do math beyond basic multiplication tables. Here are some shortcuts to help you do more math in your head.
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X% of Y = Y% of X. You can always swap those percentages if the calculation is easier the other way around. So 68% of 25 = 25% of 68 = 68/4 = 17.
That makes a lot of calculations easier, once you know the percentages that equal basic fractions:
Mental subtraction is easiest when you can subtract each digit without having to borrow from the next place. If the second number has larger digits than the first, things get more complicated. To avoid borrowing from places, you want to get rid of those larger digits. Here’s how:
