Question

Jenny began her pizza delivery route with 1/2 of a tank of gas in her car. When she made it back to the pizzeria, 2/5 of a tank of gas was left. How much gas did Jenny use?

Answer 1

To find the amount of gas Jenny used we need to subtract the amount with which she started the delivery route and the amount with which she ended. We have:

$\text{left}=\frac{1}{2}-\frac{2}{5}$

To subtract two fractions with different denominators we need to calculate the LCM between the two and adjust the numerators accordingly. In these case since both denominators are prime numbers we can simply multiply them to find the LCM. We have:

$\begin{gathered} \text{left}=\frac{1\cdot5-2\cdot2}{10} \\ \text{left}=\frac{5-4}{10} \\ \text{left}=\frac{1}{10} \end{gathered}$

Jenny used 1/10 of a tank of gas.