99% of people don't know this secret math trick

There's a simple trick to calculate percentages that you probably did not learn in school!

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[kevinderoo]

Alternate title: *People% of 99 don't know this secret math trick*

[denelson]

Convert 4% to 4/100 and rearrange the numbers to get an easier calculation: 4 x 75 is 300, then divide that by 100 to get 3.

[Mobyus]

4% of 75?
Well 1% of 75 is .75. Multiply that by 4, you get 3.

32% of 25?
10% of 25 is 2.5. Multiply that by 3, you get 7.5.
1% of 25 is .25. Multiply that by 2 you get .5.
Add together you get 8.

[tmontero]

Math has always been challenging to me. The content of this video is very helpful to me as I was not taught this in elementary school like some of you have commented. At the age of 70, I continue to struggle with many mathematical concepts and appreciate this video. I'm a whiz in other subjects, but math tests used to make me cry with frustration in school. I spent 10x the amount of my study time with math as my other subjects and put forth my hardest efforts just to pass tests. Any shortcuts are valuable to me. Thank you.

[joepiazza]

For more complicated things because x% of y is the same as (x/100) * y you can simply multiply x and y and divide by 100. Usually much easier.

[barttemolder]

Instead of swapping the % you can also multiply one side with a handy number and divide the other side with the same number to simplify it.
18% of 50 (/2, *2) = 9% of 100 = 9. 4% of 75 (/4, *4) = 1% of 300 = 3. 32% of 25 (/4, *4) = 8% of 100 = 8. 15% of 20 (/5, *5) = 3% of 100 = 3.
Even 23% of 52 (*4, /4) = 92% of 13 = 100% of 13 - 8% of 13 = 13 - 1.04 = 11.96.

[kenpeters]

I’m a CPA. I graduated with my undergrad degree in 1969 (Accounting major, econ minor). The very first year, amongst other math, we had an extremely simple Business Math course… rule of 72s, transpositions divided by 9 and all that stuff. It was a one semester course.nLearned so do many tricks. This commutative function was learned in high school algebra but it was reiterated in this easy business math course.

[CornerCaseGirl]

I have considered myself a math nerd my whole life (over 70) and never realized this until my PhD son pointed it out to me a few weeks ago. Sure--seems simple ... once you KNOW it! Thx, Presh!

[terydn]

I was lucky I went to school in the 60 and 70’s. We were taught this at age 8 or so. Never forget these basics.

[Typical.Anomaly]

I just multiply them and put the decimal point in the most logical spot.

[ArabianShark]

We live in an age where people, who were taught the commutative property of multiplication in school, need to be told by the press about the commutative property of multiplication.

[underscore]

i think its not so much a trick as it is an understanding of sufficient depth of what x% of y means. as u showed its literally just x/100*y which u can write any way u want that makes it ez for u to calculate: like y/100*x or yx/100.
from my experience its that people dont exactly understand that 4% means 4 out of 100 ect.

[Eduarodi]

When I went to school in the seventies, we were taught what "x% of y" actually implied and how to calculate it with the formula xy/100. We also knew that xy=yx. I would definitely grab a calculator now to get some percentage. The problem are those who know the right keys in the calculator to get the result, but have no idea what they're doing or why.

[claudiomarvel]

An easy "trick" that only works for a handful of numbers that are either multiple of 10 or 25 ain't much of a trick, that's just an obvious shortcut. Internet blows out for this means no one takes a proper look at their math. Okay, that'd be faster for "37% of 20%", what about "33% of 39" or "21% of 44"?

[UODZU-P]

Haven't watched the vid yet but I'm betting its about reversing percentages. 4% of 75 is 75% of 4 - so 3.
Edit: yep

[Ultranger]

Approx. 8 billion% of 99 people don’t know this secret math trick.

[Snarkapotamus]

I never learned this in school, but knew it anyway...if the number is less than 100, put a decimal point in front of it and multiply by the other number. Example: 5% of 63 = 5 x .63 = 4.5. If it's more than 100, divide the number by 100 and multiple. Example: 7% of 130 = 7 x 1.30 = 9.1

[Osprey]

I did the thumbnail example by dividing 100 by 4 to get 25, then dividing 75 by that to get 3. That took only 5 seconds or so, but simply switching the numbers and taking 75% of 4 is even faster. I'll have to remember that. I think that I learned it at some point long ago, but obviously forgot it.

[himneeshchowdhary]

Another problem with this trick is that it is only applicable if in x% of y, y<100.

so, the anomaly starts like 25% of 1000. You won't be doing 1000% of 25 then.

it's cool to know you can switch the x and y in x% of y, but it does not seem to be very useful. It is only useful to speed up calculations which are already pretty easy to do in head like the examples you gave in this video.

Thanks for sharing!

[darrelldixon]

Years ago, I worked out an easy conversion for Centigrade to Fahrenheit that isn't too difficult to do in your head. I am wondering if this procedure is well known.

The procedure is to double C, subtract 1/10 of the doubled amount and add 32.

Example:

C=100:

2C=200

2C-(2C/10)=180

2C-(2C/10)+32=212

This works for the following reason:

F=C(9/5)+32

But:

C(9/5) = 9C/5

9C/5 = 18C/10

18C/10 = (20C-2C)/10

(20C-2C)/10 = (20C/10)-(2C/10) = 2C-(2C/10)

Therefore:

C(9/5) = 2C-(2C/10)

And:

F=2C-(2C/10)+32