Change the fraction to a percentage first. It'll be easier.
1/5 is equal to 20%.
"What is 20% of 75?" is now the question.
Now, we set up the problem.

The part will be 'x', since it is unknown, and 75 is the whole. 20% (1/5) will be the percent, obviously.
Here's a tip: If you can't find what the whole is, it usually follows 'of'.
Fill in the blanks.

15 is 1/5 of 75.