AMC 8 1992 Problem 18

On a trip, a car traveled $80$ miles in an hour and a half, then was stopped in traffic for $30$ minutes, then traveled $100$ miles during the next $2$ hours. What was the car's average speed in miles per hour for the $4$-hour trip? $\text{(A)}\ 45 \text{(B)}\ 50 \text{(C)}\ 60 \text{(D)}\ 75 \text{(E)}\ 90$