1) step 1 the way you calculate this is by dividing the number in question by the number which comes after the "of"
So:
x percent of y is:
\(\frac{x}{y}\)
a percent of b is:
\(\frac{a}{b}\)
and for us:
x percent of y is:
\(\frac{15}{75}\)
we can also simplify it:
\(\frac{1}{5}\)
for this i divided both numbers by 15.
2) for the second step you multiply the fraction by 100%, ,which is 1: you can always multiply by 1!
\(\frac{1}{5}*100%\)=20%
because 100/5=20
was is clear? let me know if you have more questions!