A) 4 km/hr
B) 6 km/hr
C) 12 km/hr
D) 18 km/hr
Correct Answer: D
Solution :
Let total distance of the journey be \[s\] and \[{{t}_{1}},\,\,{{t}_{2}}\]and \[{{t}_{3}}\] be time taken for respective journeys, then \[{{t}_{1}}=\frac{s}{3\times 10},\,\,{{t}_{2}}=\frac{s}{3\times 20},\,\,{{t}_{3}}=\frac{s}{3\times 60}\] \[\therefore \]Total time\[t={{t}_{1}}+{{t}_{2}}+{{t}_{3}}\] \[=\frac{s}{30}+\frac{s}{60}+\frac{s}{180}=\frac{10s}{180}=\frac{s}{18}\] \[\therefore \]Average velocity\[=\frac{s}{t}=\frac{s\times 18}{s}=\mathbf{18}\,\,\mathbf{km/hr}\]
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