Keenan began his pizza delivery route with \( 3 / 5 \) of a tank of gas in his car. When he made it back to the pizzeria, \( 3 / 10 \) of a tank of gas was left. How much gas did Keenan use? Write your answer as a fraction or as a whole or mixed number.

To find out how much gas Keenan used, we can subtract the amount of gas left from the amount he started with. Keenan started with \( \frac{3}{5} \) of a tank of gas and ended with \( \frac{3}{10} \) of a tank. First, we need to find a common denominator to perform the subtraction. The least common multiple of 5 and 10 is 10. Now, we can convert \( \frac{3}{5} \) to have a denominator of 10: \( \frac{3}{5} = \frac{3 \times 2}{5 \times 2} = \frac{6}{10} \). Now we can subtract: \[ \frac{6}{10} - \frac{3}{10} = \frac{3}{10}. \] Thus, Keenan used \( \frac{3}{10} \) of a tank of gas.