a) 60 km/hr
b) 30.5 km/hr
c) 57 km/hr
d) 67.2 km/hr
Answer: d
Explanation:
$$\eqalign{
& {\text{Average speed}} \cr
& = {\frac{{2xy}}{{ {x + y} }}} \,\,{\text{km/hr}} \cr
& x = 84{\kern 1pt} \,{\text{kmph}} \cr
& y = 56\,{\kern 1pt} {\text{kmph}} \cr
& {\text{Average speed}} \cr
& = {\frac{{ {2 \times 84 \times 56} }}{{ {84 + 56} }}} \cr
& = 67.2\,{\kern 1pt} {\text{kmph}} \cr} $$
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