The cheat was is to punch it in a calculator and multiply the answer by 100. But let's actually figure it out.
Get the denominator to a factor of 100.
The most convenient one for this is 5, since that's also a factor of 15.
Reduce the fraction with 3/3 (divide both the numerator and denominator by 3, in case that's unclear), and you get 3/5.
Multiply both by 20, and you get 60/100.
60%
