3/8 is x/100 (x/100 is essentially what percent is)
so, in order to figure out what this is, you can set the two fractions equal to each other.
3/8=x/100
(12.5/12.5)(3/8)=x/100 <----- multiplying by 12.5/12.5 is the same thing as 1, but the denominator will be 100, so it is possible to see what x is equal to.
37.5/100=x/100
x=37.5
therefore, 3/8=37.5%