A ship can row a certain point along with the downstream and return the same point. What was the average speed of the ship in the whole journey, if the speed of ship in still water and current are 10 km/hr and 5 km/hr respectively?

Option 4 : 7.5 km/hr

Given,

Speed of ship in still water = 10 k/hr

Speed of current = 5 km/hr

Formula:

Average speed = Total distance/total time

Upstream speed = speed of boat - speed of current

Downstream speed = Speed of boat + speed of current

Calculation:

Upstream speed = 10 - 5 = 5 km/hr

Downstream speed = 10 + 5 = 15 km/hr

Let distance covered by upstream and downstream be 15 km

Time to cover 15 km distance upstream = 15/5 = 3 hrs

Time to cover 15 km distance downstream = 15/1 = 1 hr

Total distance = 15 + 15 = 30 km

Total time = 3 + 1 = 4 km

∴ Average speed = 30/4 = 7.5 km/hr