4 Answers

If A is a number.
15% of A = A x 15 / 100 = Answer
Example:
15% of 20 = 20 x 15 / 100 = 3

Here is my tip for finding 15% when trying to calculate a money tip for service:
Take the total of your bill, and move the decimal place over one to the left for 10%. Divide that number by half, and add it to the 10% you calculated. The sum is a rounded 15% of your total bill.
For example:
Your total bill is: $40.60
Move the decimal and get: $4.06
Divide that by half: $2.03
Add the two together: $6.09
Then finally add that to your original total, and you get:
$40.60 + $6.09 = $46.69
To calculate 20% tip for good service, divide the original 10% by two, and add that to the 15%.
OR
Calculate 10% and double that.

To find 15% of a number we need to multiply that number with 15/100, that is .15. If we need to find 15% of 20 we have to multiply 20 with 0.15, so we get 20*.15= 3
Suggested reading…
Express one quantity as a percentage of another
Thjis is fairly simple to do once you have got the hang of percentages. Effectively we need to divide one number by the other and then make that fraction into a percentage by multiplying by 100. This is probably best shown in an example.
Example 1
Q. “Hannah scored 33 out of 40 marks in her History exam. James did a slightly different assessment, and scored 60 out of 70 marks. Hannah remarks that James did better on his exam. James agrees, pointing out he did nearly twice as well as Hannah!”
Are their statements accurate? What's the first thing we need to do before we can decide? We must begin with working out how to express one quantity as a percentage of another!
A. Well they both did different assessments, so we need something to compare them fairly before we can say for sure. This is really, really easy to do. All we need to do is the following:
1) Just divide each of their scores, by the total for the paper.
Hannah: 33/40 = 0.825 & Junaid: 60/70 = 0.857
Straight away, you can see James did perform better overall, but certainly not by as much double Hannah’s performance.
But, because decimals like this are awkward to work with, we came up with an idea of expressing the decimal as a percentage (you can also do this with fractions)–
2) Multiply numbers like that by 100, and we’ll get a good measure that will reduce the amount of decimals we have to work with. We then call that number a ‘percentage’.
Hannah: 0.825 x 100 = 82.5% & Junaid: 0.857 x 100 = 85.7%  86%
For an assessment, that would be their percentage scores.
So to express one number as a percentage of another
 Divide the first number by the second, and then
 Multiply the result by 100.
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