Distance between two stations A and B is 778km. A train covers the journey from A to B at 84 km per hour and returns back to A with a uniform speed of 56 km per hour. Find the average speed of train during the whole journey.
A. 60 km/hr
B. 30.5 km/hr
C. 57 km/hr
D. 67.2 km/hr
Answer: Option D
Solution(By Examveda Team)
The required average speed given by the formula,$$\eqalign{ & {\text{Average speed}} \cr & = {\frac{{2xy}}{{ {x + y} }}} \,\,{\text{km/hr}} \cr & {\text{Where}}, \cr & x = 84{\kern 1pt} \,{\text{kmph}} \cr & y = 56\,{\kern 1pt} {\text{kmph}} \cr & {\text{Average speed}} \cr & = {\frac{{ {2 \times 84 \times 56} }}{{ {84 + 56} }}} \cr & = 67.2\,{\kern 1pt} {\text{kmph}} \cr} $$
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Related Questions on Average
A. 125 km/hr
B. 75 km/hr
C. 135 km/hr
D. 120 km/hr
Time T1 = 778/84, T2 = 778/56
Total Time = T1+T2 = 778(1/84+1/56), Total Distance = 2*778
Average speed = Total Distance/Total Time
= 2*778/778(1/84 + 2/56)
= 2/(84+56)/84*56
= 2*84*56/(84+56)
= 67.2