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