a) 35 km/hr
b) 37 km/hr
c) 37.5 km/hr
d) 40 km/hr
Answer: c
Explanation: Average speed
$$\eqalign{
& = \left( {\frac{{2xy}}{{x + y}}} \right){\text{ km/hr}} \cr
& = \left( {\frac{{2 \times 50 \times 30}}{{50 + 30}}} \right){\text{ km/hr}} \cr
& = 37.5{\text{ km/hr}} \cr} $$
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