10 Little Hacks From Math Nerds That I Tried And Memorized Because They're So Easy And Useful
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."
"And then if you do figure it out, to subtract $105 from $150??), take that 10% and multiply it by what you’re GOING TO PAY. So in this case: $15 X 3 = $45."
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.
"If you were to find the area of a pentagon, octagon, etc. instead of using perimeter x apothem x 1/2, you can count the number of sides, ex: a pentagon 5/2 (the amount of sides divided by 2) x length of one side x apothem. Works every time."
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.
2: Even numbers.
3: The sum of the digits is divisible by 3 (516 is divisible by 3 bc 5+1+6=12, and 12 is divisible by 3. And it’s recursive and do another round.
4: Literally nothing matters except for the final 2 digits—those have to be divisible by 4. 5,6715,324 is divisible by 4 bc 24 is divisible by 4.
5: Ends in 5 or 0
6: Divisible by 3 AND even.
9: Same rule as 3’s, but now it’s if the sum of the digits is divisible by 9. 918 is divisible by 9 bc 9+1+8=18 and 18 is divisible by 9.
10: Ends in 0.
6."Percent can be tricky, especially if you don’t have a calculator, BUT, they’re 'reversible' (not proper math term)."
"Say you needed to know 12% of 25. That’s not the easiest math to figure out in your head. However 25% of 12 is much easier to calculate because that is just 1/4 of 12.
25% of 12 is 3. And 12% of 25 is 3."
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!
