If a boat goes upstream at a speed of 21 km/h and comes back the same distance at 28 km/h. What is the average speed (in km/hr) for the total journey?
Let say total distance is d km.
So, average speed=2d/(d/21+d/28) km/hr.
=(2d×84)/(4d+3d) km/hr
=24 km/hr.
B is correct choice.
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