How do you add 15 percent to a cost?

Answer 1

There are 2 ways:

The most basic is to convert the percent to a decimal by dividing it by 100.

15% / 100 = .15

Then multiply that decimal to the cost, let's say it's $50.

$50 * .15 = $7.50

Then add that value to your original cost: $50 + $7.50 = $57.50

The "quick and dirty" way (if you don't actually need to know the amount of percentage and just need to add) is to do this:

Convert the percentage to a decimal and then add 1:

15% / 100 = .15

.15 + 1 = 1.15

Then just multiply that to your cost! $50 * 1.15 = $57.50

Also ...

if you'd like to do 15% in your head (say for a tip) that can be easy too:

15% is 10% plus 5%. (true)

10% of something is twice as much as 5% of something. (also true)

taking 10% of something is easy (just move the decimal point to the left 1 place)

Example:

If you need to know 15% of $42.60 and you don't have a calculator, then multiply that price times .10 (that's 10% as a decimal).

All you need to do is move the decimal place once to the left. 42.60 becomes 4.260

And if 5% is half of 10%, then take half of that amount:

4.260 * half = 2.13.

Now just add the two together:

4.26+2.13 = 6.39.

15% of $42.60 is $6.39.

Even simpler advice ... if you're needing to calculate 15% in your head for the specific reason of a tip, here's what I do ...

Consider the example of $42.60. Round down to $40.00. 10$ would be 4$ and half of that would be $2 for a total of $6.00.

A server isn't going to be upset if they're short 39 cents. but if you want to make sure you do right by them ...

Round down to the nearest 10 dollars, figure the tip (6$) and then add 50 cents if the total was X5.00 or lower. Or if it was X5 or higher, add $1.00.