This problem can easily confuse people who are easily confused.
It makes absolutely no difference how many of each kind of coin he had. The
only important fact is that he had (16 + 28 + 32 + 4) = 80 coins all together.
He sold 15% of them. ' 15% ' means ' 0.15 ', and ' of ' means ' times '.
0.15 x 80 = 12 coins sold.
