If speed of stream is 20% of the speed of boat in still water and it covers 120 km upstream in 150 minutes, then what is the downstream speed of the boat?
Answer & explanation
Correct answer: option 2
The speed of steam = 20% of the speed of boat .
The boat covers 120 km upstream in 150 minutes
Let the speed of boat = a km/hour
= The speed of stream is 20% of a = \(\frac{a}{5}\)
= The speed of boat in upstream = a - \(\frac{a}{5}\) = \(\frac{4a}{5}\)
And the speed of boat in downstream = a + \(\frac{a}{5}\) = \(\frac{6a}{5}\)
Now, according to question
120 = \(\frac{4a}{5}\) × 150/60
= 120 = 2a
= a = 60
So, the downstream speed of boat = 6 × \(\frac{60}{5}\) = 72 km/hour