Correct Answer - Option 3 : 26 km/hr
Given∶
Ratio of the speed of the boat in still water to the speed of the stream = 7 ∶ 5
Time to travel distance of 156 km downstream = 1 hour
Formula used ∶
Speed upstream = (u – v)
Speed downstream = (u + v)
Where, u = Speed of boat in still water
v = Speed of stream
Calculation∶
Let the speed of boat in still water be 7x and the speed of stream be 5x
Speed while travelling downstream = u + v
⇒ 7x + 5x = 12x
Time taken to travel 156km downstream is 1 hr
\(\Rightarrow 1 = \frac{{156}}{{12x}}\) ---(1)
⇒ x = 13
Difference between speed of boat in still water and the speed of stream
⇒ 7x – 5x km/hr = 2x km/hr
Put the value of x = 13 in 2x, we get
⇒ 2 × 13 = 26 km/hr
∴ The difference between the speed of boat in still water and speed of stream is 26 km/hr
