Finding a percentage of a number

Mariner · 2010-12-20 17:49:03

I'm brushing up on my 'simple' maths before I go a do a placement test and I know for a fact that there are a number of questions that ask for a percentage of a number or quantity of something. I was wondering if I can have some tips on how to do these types of questions in my head or any 'tricks of the trade' if any.

The structure and style of questions asked are as below.
Find 25% of 5 hours.

Find 45% of 70 meters.

Find 5% of $5

Thanks for the help:D

bobbym · 2010-12-20 17:59:50

H Mariner;

Welcome to the forum.

Find 5% of $5

I do this type like this, move the decimal 1 place to the left:

Then divide by 2.

stapel · 2010-12-21 13:33:53

Mariner wrote:

...I know for a fact that there are a number of questions that ask for a percentage of a number or quantity of something. I was wondering if I can have some tips on how to do these types of questions in my head or any 'tricks of the trade' if any.

The general equation for is as follows:

. . . . .(this) is (some percent) of (that)

...where "is" means "equals" and "of" means "times".

You'll always be given two pieces of information. Plug those in, and solve for whatever is left, which will be what they're wanting.

Last edited by bobbym (2013-03-18 01:20:20)

John E. Franklin · 2010-12-28 04:07:02

I'll do 45% of 70.
double 45 to 90 while at the same time you half 70 to 35.
So now the problem is:
90% of 35.
This is easier. But how?
Remember that 90 is (100 - 10), so do this:
35 - 3.5 = answer
Right?
So 31.5.
Otherwise you could do it out in your head:
45% of 70.
.45
x7
2.80 + .35 = 3.15 but now I did 7 not 70, so 3.15 becomes 31.5 answer.

John E. Franklin · 2010-12-28 04:08:29

Now 25% of 5 hours is easy.
For 25% always half twice in a row.
half of 5 is 2.5
half of 2.5 is 1.25
Answer is 1.25

John E. Franklin · 2010-12-28 04:10:06

If someone wants 20% of 120.
Then remember the 20% rule (divide by 5 rule).
Double and divide by ten (this works because 2/10 = 1/5)
So double 120 is 240 and then 240/10 is 24.
Answer 24.

John E. Franklin · 2010-12-28 04:14:52

Now 80% of 136.
For 80% double 3 times and move over decimal place (2^3/10)
double 136 = 272
double 272 = 544
double 544 = 1088
move dot over left = 108.8

Another way is to subtract off 20% from 100% to get 80% using the 20% rule:
double 136 = 272, move dot over left, get 27.2, and subtract: 136 - 27.2 = 108.8
Same answer.

John E. Franklin · 2010-12-28 04:17:02

Now 75% of 144.
For 75%, think 3/4 because dividing by 4 is easy, just half twice.
Half of 144 is 72.
half of 72 is 36.
3 times 36 is 108 because (3 x 33) = 99 and 3 x 3 = 9 and 99 + 9 = 108.
(36 = 33 + 3)
Answer is 108.

John E. Franklin · 2010-12-28 04:18:59

Another one to know is 67 times 300% because it is so close to 66.6666666%
67 + 67 = 134
134 + 67 = 190 + 11 = 201
Answer 201.
So 33.33333% of 201 is 67 also.
Note that 2 + 0 + 1 = 3, the 3 rule for division.

John E. Franklin · 2010-12-28 04:21:37

Also research The Trachtenberg Speed System of BASIC mathematics, though the techniques take time to learn.

Monox D. I-Fly · 2016-08-17 15:44:30

Speaking of Trachtenberg system, does it have any method for speed subtraction?

Math Is Fun Forum

#1 2010-12-20 17:49:03

Finding a percentage of a number

#2 2010-12-20 17:59:50

Re: Finding a percentage of a number

#3 2010-12-21 13:33:53

Re: Finding a percentage of a number

#4 2010-12-28 04:07:02

Re: Finding a percentage of a number

#5 2010-12-28 04:08:29

Re: Finding a percentage of a number

#6 2010-12-28 04:10:06

Re: Finding a percentage of a number

#7 2010-12-28 04:14:52

Re: Finding a percentage of a number

#8 2010-12-28 04:17:02

Re: Finding a percentage of a number

#9 2010-12-28 04:18:59

Re: Finding a percentage of a number

#10 2010-12-28 04:21:37

Re: Finding a percentage of a number

#11 2016-08-17 15:44:30

Re: Finding a percentage of a number

Board footer