The speed of boat in upstream is 3/5 th of the speed of boat in downstream. The speed of boat in still water is 6 km/hr more than the speed of stream. Then find the total time taken by boat to cover 25 km in downstream and 15 km in upstream?

Correct option is D

Information given:
The speed of boat in upstream is 3/5 th of the speed of boat in downstream
The speed of boat in still water is 6 km/hr more than the speed of stream
Formulas Used:
Still water speed of boat = (speed of boat in downstream+ speed of boat in upstream)/2
Speed of stream = speed of boat in downstream - speed of boat in upstream)/2
Explanation:
Let the speed of boat in downstream be 5x km/hr
And the speed of boat in upstream be 3x km/hr
Speed of boat in still water = (5x+3x)/2 = 4x km/hr
Speed of stream = (5x–3x)/2 = x km/hr
ATQ,
3x = 6
x = 2
Required time = 25/10+15/6 = 5 hours