Correct Answer - Option 1 : 2.5 hours
Given:
A boat goes 7 km upstream in 2 hours and same distance downstream in 40 minutes
Concept used:
Boat and Stream.
Calculation:
Let the speed in still water be x km/hr
Speed of Stream be y km/hr
Speed Downstream = (x + y) km/hr
Speed Upstream = (x - y) km/hr
Boat covers 7 km upstream in 2 hours
x - y = 7/2
Boat covers 7 km downstream in 40 minutes
x + y = \(\frac{{7 \times 3}}{2} = \frac{{21}}{2}km/hr\)
Equation 1 = 2x - 2y = 7
Equation 2 = 2x + 2y = 21
Solving both equations we get
x = 7 km/hr
Time required to travel \(17\frac{1}{2}km\)
Time = \(\frac{{35}}{{2 \times 7}} = \frac{5}{2}\)
Time taken = 2.5 hours