Question

15% of what number is 48?

Answer 1

The answer would be 320. all you have to do is set up a proportion.
that looks like 15 over 100 and 48 over n ( n is the number we don't know yet )
and cross multiply to get; 15n and 100*48=4800
now you would have to divide both 15 and 4800 by 15 
to get; 15n/15=n and 4800/15=320
so you would get n=320

Answer 2

15% of 320 is 48

Change the percentage to a decimal by dividing it over 100:
$\frac{15}{100} = 0.15$

Divide 48 by 0.15 to get the answer:
$\frac{48}{0.15} = 320$