Recently, I asked the BuzzFeed Community for their best math trick. I’m not talking quantum physics here, but just, you know, an easy way to figure out how much butter I need if I’m doubling a recipe. The answers are remarkably useful, so without further ado, here they are!

1.

“Sales! If something is 70% off, you are paying 30%. If something is $150 on sale for 70% off rather than figure out 10% and multiply by 7 and subtract that from $150.”

2.

Find the area of an equal-sided shape by multiplying the center an edge, the length of one side, and half the amount of sides.

3.

“Very small and is just a parlor trick, but if you multiply any double-digit number by 11, then you just add the two numbers of the double-digit together and put that in the middle.”

4.

“The one that a lot of people find helpful, especially if you cook or bake a decent bit, is the following: multiplying fractions is no problem–top times top (numerator multiplied by numerator) over bottom times bottom (denominator multiplied by denominator).”

5.

Divisibility rules are wildly useful.

6.

“Percent can be tricky, especially if you don’t have a calculator, BUT, they’re ‘reversible’ (not proper math term).”

7.

“To divide by 5 in your head, double the number, then move the decimal point once to the left (divide by 10)”

8.

“When I add, I break it down and do easy first, then hard. So for 25 + 56, I do 25 + 50 = 75 + 6 = 81”

9.

“The nines multiplication hand trick!”

10.

“It’s easier to add numbers that end in 0, so kind of a different take on another comment…This especially helps with mental math. You could even take it a next step by saying 39 is “1” away from 40. So instead of adding 73+39, you add 70+40, which is 110, then subtract the “1” and get the final answer of 109.”

Do you have a math tip you swear by? Tell us all about it in the comments below!