a) 25 km/h
b) 28 km/h
c) 30 km/h
d) 33 km/h
Answer: a
Explanation:
$$\eqalign{
& {\text{Let the total distance be 100 km}}. \cr
& {\text{Average speed}} \cr
& = \frac{{{\text{total}}\,{\text{distance}}\,{\text{covered}}}}{{{\text{time}}\,{\text{taken}}}} \cr
& = \frac{{100}}{{ { {\frac{{30}}{{20}}} + {\frac{{60}}{{40}}} + {\frac{{10}}{{10}}} } }} \cr
& = \frac{{100}}{{ { {\frac{3}{2}} + {\frac{3}{2}} + 1 } }} \cr
& = \frac{{100}}{{ {\frac{{ {3 + 3 + 2} }}{2}} }} \cr
& = \frac{{ {100 \times 2} }}{8} \cr
& = 25\,\text{kmph} \cr} $$
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