99% Of People Don't Know This Math Secret

This little trick makes math calculations much easier.

Sources/references (March 2019)

Updated reference June 2021
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Connect with me

[ln-physics]

The formula which helps me a lot in solving problems is
*a+b=b+a*

[marcusbrsp]

Nice trick!
Previously, I always found out what 1% is, which often requires very little thought, then I simply multiply.
Example *4% of 75*
1% of 75 = 0, 75
0, 75*4 = 3
But this new trick in this video is pretty nice.

[TheMirolab]

The way I do low percentages is just take 1% of the number, and then add them up.... In this case: 1% of 75 is 0.75. Now just add 0.75 up 4 times = 3. This works pretty easy up to around 6%.

[grahammcfadyenhill]

I learned this as a teenager in 1961. I worked in a clothing store with crank style cash registers...not even electric...and we had to calulate sales tax. All of the other staff were older adults. Percent defeated them all. They were, to an old man and woman, math-challenged. I tried to tech them all this trick, succeeded with some. Ended up as a high school math teacher for 35 years.

[gnugo]

Fun fact: People% of 99 don't know this math secret.

[mjones]

I've been teaching this percent trick for ages... this is the first time I've seen in presented anywhere. Another of my favorites is doubling a five and halving an even. Examples:
35% of 60 = 70% (double the multiple of five) of 30 (half the even number) = 21.
120% of 55 = 60% (half the even number) of 110 (double the multiple of five) = 66.

[portobellomushroom]

today on MindYourDescisions: the commutative property of multiplication
tomorrow on MindYourDescisions: calculating the surface volume of a higher dimensional spheres

[Nathan]

I didn’t know this, but I always use the 10% & 1% tricks to figure stuff out. For 10% of something you just move the decimal over 1 space. For 1% you move it over 2. Then you can adjust from there. So for 4% of 75? 75.0, move the decimal over 2, and you get 0.75. That’s 1%. Times that by 4. Or if you want 5% of it, you move the decimal over 1, which would be 7.5 and that’s 10%. Cut it in half and that’s 5%. But this is definitely easier for a lot of situations so I’m glad you shared it.

[prakashjha]

I did 1% of 75 is 0.75 so 4% will be 0.75×4 that's equal to 3

[nickvds]

I actually had a teacher who told me this trick, all fun and games, until it’s 23% of 52

[Mr.Titanium]

Harder when the percentage isn’t easily divisible though

[marknasia]

This is great for people that know their “times/multiplication tables” by memory. The real problem, as great as this trick is, many have been using calculators in school for so long, they never memorized their multiplication tables. Which means they already rely on a calculator to perform the function

[philbow]

There’s three kinds of people, those who can count and those who can’t.

[yeltsin]

My dad who came from Yugoslavia but had troubling the English language and worked in a factory his whole life taught me mental math tricks. When I saw the original question I knew it was 3 without even thinking about it. My dad could calculate the grocery bill with taxes before the cashier would be done. He explained it and was so simple. I’ve taught my girls the same trick. It helps with everyday stuff.

[X-Gen-]

I could've used that in my first year of high school. 34 years ago. lol

[dellcoc]

I was actually taught this in the 3rd grade and have used it ever since. One of my favorite teachers in all school.

[tahirul]

Huh, i knew X and Y were commutable in (X/Z)×Y but I never thought to apply it to percentages like that. That's a handy little trick.

[allisonjustice]

Just remember "of" is "multiplication" and your life will be easier :)

[cmart]

I think that the most important thing is actually understand what the word "percent" means, which probably 100% of people do not know. "Percent" means "per one hundred". This means that 5% is 5 per one hundred or 5 divided by one hundred since "per" in latin is "divide by" in this context.

Hence, 4% of 75 is nothing but 0.04*75 which is 3. If you think this is hard, simply multiply by 4 and you get 300. Now just place de dot two decimal places from right to left.

Bottom line is: if you do not understand the underlying reasoning, any trick is likely to confuse you further because you still do not understand what you are trying to do and achieve.

[Yiryujin]

This is one of those cases where students are taught the rules of multiplication but doesn't actually know how to utilize it except at most rudimentary of levels.