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.
[tex] \frac{part}{whole} = \frac{percent}{100} [/tex]
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.
[tex] \frac{x}{75} = \frac{20}{100}\\\\cross\ multiply\\\\75\times20=1500\\100\times x=100x \\\\1500=100x\\x=15[/tex]
15 is 1/5 of 75.
