jmc

If Terrell lifts two 20-pound weights 12 times, he lifts a total of $2\cdot 12\cdot 20=480$ pounds of weight. If he lifts two 15-pound weights instead for $n$ times, he will lift a total of $2\cdot 15\cdot n=30n$ pounds of weight. Equating this to 480 pounds, we can solve for $n$: $$\begin{array}{rl}30n& =480\\ \Rightarrow n& =480/30=\boxed{16}\end{array}$$