Correct Answer - Option 1 : 5.5 kmph

**Given:**

u/s = 20 km in 10 hours

Ds = 72 kms in 8 hours

**Formula used:**

If speed in upstream in X kmph and downstream is Y kmph

Speed in still water = (1/2)(X + Y) kmph

Rate of stream = (1/2)(X – Y) kmph

**Calculations:**

Speed of upstream and downstream

⇒ Speed of upstream = 20 / 10

⇒ Speed of upstream = 2 kmph

⇒ Speed of downstream = 72 / 8

⇒ Speed of downstream = 9 kmph

⇒ speed in still water = (1/2)(2 + 9)

⇒ Speed in still water = 11/2 kmph

**∴**** Speed in still water is 5.5 kmph**