Question
The speed of a boat while going downstream is 24 km/h, and
while going upstream, it is 16 km/h. If the boatman takes 12 hours to cover (m + n) km upstream when the stream flows at twice its normal speed, find the average of m and n.Solution
ATQ,
Let speed of boat in still water and stream speed be B km/hour and s km/hour, respectively.
So, B + s = 24 … (i)
B - s = 16 … (ii)
Solving equation (i) and (ii), we get,
B = 20 km/hour
s = 4 km/hour
According to the question,
(m + n)/(B - 2s) = 12
(m + n) = 12 × (20 - 2 × 4) = 12 × 12 = 144 km
Therefore, average value of m and n = (m + n)/2 = 144/2 = 72 km
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