Multiple choice

A boat takes $7$ hours to travel $30km$ upstream and $28km$ downstairs. It takes $5$ hours to travel $21km$ upstream and to return back. Find the speed of the boat in still water.

  1. $10$ $km/hr$
  2. $20$ $km/hr$
  3. $14$ $km/hr$
  4. $6$ $km/hr$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let the upstream and downstream speeds be u and d. From 21/u + 21/d = 5 and 30/u + 28/d = 7, we obtain u = 6 km/hr and d = 14 km/hr. The speed in still water is the average, (6 + 14)/2 = 10 km/hr.