a) 125 km/hr
b) 75 km/hr
c) 135 km/hr
d) 120 km/hr
Answer: d
Explanation:
$$\eqalign{
& {\text{Average speed}}, \cr
& = \frac{{ {2 \times x \times y} }}{{ {x + y} }} \cr
& = \frac{{ {2 \times 100 \times 150} }}{{ {100 + 150} }} \cr
& = \frac{{ {200 \times 150} }}{{250}} \cr
& = 120\,{\text{km/hr}} \cr} $$
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