A boat covers 780 km downstream in 15 hours. If the speed of the boat in still water is decreased by 20%, then the time taken by the boat to cover 624 km upstream in 24 hours. Find the speed of the stream (In km/hr).

Correct option is D

Information Given:
A boat covers 780 km downstream in 15 hours.
The speed of the boat in still water is decreased by 20%.
The time taken by the boat to cover 624 km upstream in 24 hours.
Formulas Used:
Downstream speed = Speed of boat in still water + Speed of current
Upstream speed = Speed of boat in still water - Speed of current
Basic Explanation:
Let the speed of boat in still water and speed of current be X km/hr and Y km/hr respectively.
Downstream speed = $$\frac{780}{15}$$​ = 52 km/h
X + Y =52 …..(I)
New speed of the boat in still water =$$X \times \frac{80}{100}$$​= 0.8X km/h
ATQ,
$$\frac{624}{24} = 0.8X - Y$$​
26 = 0.8X – Y…..(II)
From I & II
X = $$\frac{130}{3}$$​

Y = $$\frac{26}{3}$$​
The speed of the stream = $$\frac{26}{3}$$​ = 8.67 km/h