a) 9 km/h
b) 4.5 km
c) 4 km/h
d) 3 km/h
Answer: c
Explanation:
$$\eqalign{
& {\text{Average}}\,{\text{speed}} \cr
& = \frac{{2xy}}{{ {x + y} }} \cr
& = \frac{{2 \times 6 \times 3}}{{ {6 + 3} }} \cr
& = \frac{{36}}{9} \cr
& = 4\,{\text{kmph}} \cr} $$
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