A. 103

B. 105

C. 110

D. 112

E. 115

Answer: A

Explanation:

Assuming total distance travelled = 2x miles

Time taken for the first “x” miles = \frac{x}{90}hours

Time taken for the second half = \frac{x}{120} hours

Average speed = \frac{Total distance}{Total time taken}

So, Average speed = \frac{2x}{\frac{x}{90}+\frac{x}{120}} = \frac{720}{7} = 102.8 mph

Closest answer is 103mph.

Shortcut:

If the distance travelled is constant, then the average speed = \frac{2ab}{a+b}

Where “a” and “b” are two different speeds.

\frac{2ab}{a+b}=\frac{2*90*120}{210}=\frac{21600}{210} = 102.8 mph

Closest answer is 103mph.

Hence the answer is A.