Practicing Success
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? |
75 km/h 72 km/h 80 km/h 64 km/h |
72 km/h |
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 |