Question

5. A car is making a 12-mile trip. It travels the first 6.0 miles at 30 miles per hour and the last 6.0 miles at 60 miles per hour. What is the car's average speed for the entire trip? A) 20 mph B) 35 mph C) 40 mph D) 45 mph E) 50 mph

Answer 1

Answer:C

Explanation:

Given

Total distance covered by car $d=12\ miles$

First it travels $d_1=6\ miles$ with $v_1=30\ mph$

then it travels $d_2=6\ miles$ with $v_2=60\ mph$

time taken during first half $t_1=\frac{d_1}{v_1}=\frac{6}{30}=0.2\ hr$

time taken during Second half $t_1=\frac{d_2}{v_2}=\frac{6}{60}=0.1\ hr$

average speed $v_{avg}=\frac{distance}{time}$

$v_{avg}=\frac{12}{0.2+0.1}=\frac{12}{0.3}=40\ mph$