A bicycle trip is $30 \mathrm{~km}$ long. Ari rides at an average speed of $20 \mathrm{~km} / \mathrm{h}$. Bri rides at an average speed of $15 \mathrm{~km} / \mathrm{h}$. If Ari and Bri begin at the same time, how many minutes after Ari finishes the trip will Bri finish?
(A) $50 \mathrm{~min}$
(B) $40 \mathrm{~min}$
(C) $30 \mathrm{~min}$
(D) $20 \mathrm{~min}$
(E) $10 \mathrm{~min}$
Riding at $15 \mathrm{~km} / \mathrm{h}$, Bri finishes the $30 \mathrm{~km}$ in $\frac{30 \mathrm{~km}}{15 \mathrm{~km} / \mathrm{h}}=2 \mathrm{~h}$.

Riding at $20 \mathrm{~km} / \mathrm{h}$, Ari finishes the $30 \mathrm{~km}$ in $\frac{30 \mathrm{~km}}{20 \mathrm{~km} / \mathrm{h}}=1.5 \mathrm{~h}$.

Therefore, Bri finishes $0.5 \mathrm{~h}$ after Ari, which is 30 minutes.

ANswer: (C)