Question

3 is 15% of what number

Answer 1

'of' means multiply
percent mean partss out of 100 so x%=x/100

translate

3 is 15% of what
3=15/100 times what
3=0.15 times what
divide both sides by 0.15
20=what

the answer is 20

Answer 2

Use the rule of 3 simple 

x mean 100%
3 mean 15%
-------------------
 x = 3*100/15 = 300/15 = 20

hope this will help you