8is 15% of what number?
15% of 18.95 is what number?
what percent of 64 is 326?
10% of what number is 40?

Q1. The answer is 53.33

The initial number is 100%. So, make a proportion:
If 8 is 15%, what number is 100%?
8 : 15% = x : 100%
15% * x = 8 * 100%
x = 8 * 100% / 15% = 53.33

Q2. The answer is 2.8425

The initial number 18.95 and it is 100%. How much is its 15%? Make a proportion:
If 18.95 is 100%, what number is 15% of it?
18.95 : 100% = x : 15%
x * 100% = 18.95 * 15%
x = 18.95 * 15% / 100% = 2.8425

Q3. The answer is 509.375%

The initial number is 64 and it is 100%. So, what percent is 326? Make a proportion:
If 64 is 100%, 326 is what percent?
64 : 100% = 326 : x
x * 64 = 326 * 100%
x = 326 * 100% / 64 = 509.375%

Q4. The answer is 400.

The initial number is 100%. Make a proportion:
If 40 is 10%, what number is 100%?
40 : 10% = x : 100%
x * 10% = 40 * 100%
x = 40 * 100% / 10%
x = 400