Correct Answer - Option 2 : 10 ∶ 9
Given∶
The speed of boat in still water is 7 km / hr.
The speed of stream is 2 km / hr.
The time taken by the man to row upstream is twice the time taken by him to row downstream
Formula used ∶
Let u and v be the speed of boat in still water and speed of stream respectively.
Speed upstream = (u - v)
Speed downstream = (u + v)
Calculations∶
Let the distance travelled upstream = x km
And the distance travelled downstream = y km
Speed upstream = (u - v) = 7 - 2 = 5 km / hr
Speed downstream = (u + v) = 7 + 2 = 9 km / hr
Time taken by the man to row upstream is = \(\frac{{\rm{x}}}{5}\)
Time taken by the man to row downstream is = \(\frac{{\rm{y}}}{9}\)
The time taken by the man to row upstream is twice the time taken by him to row downstream ∶
\(\Rightarrow \frac{x}{5} = \left( {\frac{y}{9}} \right) \times 2\)
⇒ x ∶ y = 10 ∶ 9
Hence the ratio of distance travelled by the man upstream to that of downstream is 10 ∶ 9.
