Let the speed of stream be ‘a’ km/hr.

Given, speed of a boat in still water is 8 km / hr. It can go 15 km upstream and 22 km downstream in 5 hours.

Relative speed of boat going upstream = 8 – a

Relative speed of boat going downstream = 8 + a

Time = distance/speed

⇒ 120 + 15a + 176 – 22a = 5(64 – a²)

⇒ 296 – 7a = -5a² + 320

⇒ 5a² – 7a – 24 = 0

⇒ 5a² – 15a + 8a – 24 = 0

⇒ 5a(a – 3) + 8(a – 3) = 0

⇒ (5a + 8)(a – 3) = 0

⇒ a = 3 km/hr