Lily has $3$ gallons of milk and gives $\frac{12}{5}$ gallons to James. What fraction of a gallon does she have left?
To do that, I need to subtract $\frac{12}{5}$ from $3$. So, $3 \times \frac{5}{5} = \frac{15}{5}$. Now, I can subtract $\frac{12}{5}$ from $\frac{15}{5}$ by subtracting the numerators and keeping the same denominator. That gives me $\frac{15}{5} - \frac{12}{5} = \frac{3}{5}$. So, Lily has $\frac{3}{5}$ of a gallon of milk left.

