AMC 10 2020a Problem 2

The numbers $3, 5, 7, a,$ and $b$ have an average (arithmetic mean) of $15$ . What is the average of $a$ and $b$ ? $\textbf{(A) } 0 \textbf{(B) } 15 \textbf{(C) } 30 \textbf{(D) } 45 \textbf{(E) } 60$