Correct Answer - Option 1 : 15

**Given:**

A boat speed in still water = 20 km/hr

Distance travelled by boat downstream = 75 km

Distance travelled by boat upstream = 75 km

Total time of travel for both upstream and downstream = 8 hours

**Formula:**

Upstream speed = (u − v) km/hr

Downstream speed = (u + v) km/hr

where “u” is the speed of the boat in still water and “v” is the speed of the stream

Speed of Boat in Still Water = 1/2 (Downstream Speed + Upstream Speed)

Speed of Stream = 1/2 (Downstream Speed – Upstream Speed)

**Calculation:**

Let the speed of the stream be x km/hr

Upstream Speed = 20 - x

Downstream Speed = 20 + x

⇒ 75/(20 + x) + 75/(20 - x) = 8

⇒ 75(20 - x + 20 + x)/(400 - x^{2}) = 8/75 = 375

⇒ 400 - x2 = 375

⇒ x = 5

The Upstream speed = 20 - 5 ⇒ 15 km/h

**∴ The upstream speed is 15 km/h.**