How long will it take to row 20 km upstream if one can row 10 km in 10 minutes in still water and the same distance in 8 minutes with the stream?

Question: How long will it take to row 20 km upstream if one can row 10 km in 10 minutes in still water and the same distance in 8 minutes with the stream?

Solution:
Let,
x be the speed of man in still water
y be the speed of stream.
∴ Speed of man x = 10/10 km/min
= 1 km/min
= 60 km/hr

In downstream in 8 min he can row 10 km
∴ In downstream in 60 min he can row (10 × 60)/8 km
= 75 km

∴ x + y = 75
⇒ y = 75 - 60
∴ y = 15
∴ Speed of stream = 15 km/hr.

Hence upstream speed = 60 – 15 = 45 km/hr.

Time taken to cover 45 km = 60 min
∴ Time taken to cover 20 km = (60 × 20)/45 min
= 26.67 min