Fifteen times the square of a non-zero number is equal to 20 times the number.

The square of a number is n^2.  So

15n^2=20n  subtract 20n from both sides

15n^2-20n=0  factor our 5n

5n(3n-4)=0, since we are told that n does not equal zero...

3n-4=0

3n=4

n=4/3