How long must a driver take to drive the final 70 miles of a trip if he wants to average 50 miles an hour for the entire trip and during the first part of the trip he drove 50 miles in 11/2 hours ?

Answer: Option A

Total distance = (57 + 50) miles = 120 miles
Average speed = 50 miles/hour
Required time of journey :
$$\begin{array}{rl}& =\left(\frac{120}{50}\right)\text{ hr}\\ & =\frac{12}{5}\text{ hr}\\ & =2\frac{2}{5}\text{ hr}\\ & \text{ = 2 hr }24\text{ min}\end{array}$$
Time taken to cover 50 miles :
$$\begin{array}{rl}& =\text{1}\frac{1}{2}\text{ hr}\\ & =1\text{ hr 30 min}\end{array}$$
∴ Remaining time :
= (2 hr 24 min - 1 hr 30 min)
= 54 min