Percent equation math question, can someone please help me out here??? Thanks?

In your own words, how would you explain how to solve a percent equation, using this given example below:
Last week, I was having a 15%-off sale on all Piranha II (80% Tungsten) soft tip darts. Out of the 42 sets of the Piranha II darts I had in stock, I sold 27 sets.
If thid would [...]