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You probably haven’t had to do longhand 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 any math beyond your basic multiplication tables. Here are some shortcuts that will help you do more math in your head.
Calculate percentages backward
X% of Y = Y% of X. You can always swap those percentages if doing the math is easier the other way around. So 68% of 25 = 25% of 68 = 68/4 = 17.
That makes a lot of calculations easy, once you’ve memorized the percentages that equal basic fractions:
- 10% = 1/10
- 12.5% = 1/8
- 16.666…% = 1/6
- 20% = 1/5
- 25% = 1/4
- 33.333…% = 1/3
- 50% = 1/2
- 66.666…% = 2/3
- 75% = 3/4
Subtract without borrowing digits
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 925-724, and then subtract that extra 10 separately: 925-724 = 201, and 201-10 = 191. There’s your answer.
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Let me think…
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James' World 2 
Feb 02, 2024 @ 05:13:24
Nice one
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Feb 02, 2024 @ 10:50:01
Thanks, math short cuts are useful!
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