Question : If a boat goes upstream at a speed of 21 km/hr and comes back the same distance at 28 km/hr, what is the average speed (in km/hr) for the total journey?
Option 1: 24.5
Option 2: 24
Option 3: 25
Option 4: 25.4
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Correct Answer: 24
Solution :
Given: Upstream speed of boat = 21 km/hr
Downstream speed of boat = 28 km/hr
For the same distance,
Average speed = $\frac{2ab}{a+b}$ where $a$ = 21 km/hr and $b$ = 28 km/hr
So, the average speed = $\frac{2\times 21\times 28}{21+28}$ = 24 km/hr
Hence, the correct answer is 24.
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