Question : Directions: The distance between the two cities is 30 km. A man goes at a speed of 30 km per hour and returns at 20 km per hour. What is his average speed?
Option 1: 25 km/hr
Option 2: 24 km/hr
Option 3: 10 km/hr
Option 4: 26 km/hr
Correct Answer: 24 km/hr
Solution :
Given:
Distance to be travelled = 30km
Speed A = 30km/hr, Speed B = 20km/hr
When the distance is constant, then the average speed = $\frac{2 × Speed A × Speed B}{Speed A + Speed B}$
= $\frac{2 × 30 × 20}{30 + 20}$
= $\frac{1200}{50}$
= 24 km/hr
So, the average speed of the man is 24km/hr. Hence, the second option is correct.
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