arithmetic - a boat can travel with a speed of 16 kmhr in still water if the rate of stream is 5 kmhr then find the time taken by the boat to cover distance of 84 km downstream - skillgun

It is very important to check, if the boat speed given is in still water or with water or against water.
Because if we neglect it we will not reach on right answer.
I just mentioned here because mostly mistakes in this chapter are of this kind only.
Lets see the question now.
Speed downstream = (16 + 5) = 21 kmph
Time = distance/speed = 84/21 = 4 hours