Multiple choice

A boat covers 4 km upstream and 4 km downstream in 1 hour. The same boat covers 5 km downstream and 3 km upstream in 55 minutes. What is the speed (in km/hr) of the boat in still water?

  1. 6.5

  2. 7.75

  3. 9

  4. 10.5

  5. 11

Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let u be upstream speed and d be downstream speed. 4/u + 4/d = 1. 3/u + 5/d = 55/60 = 11/12. Multiply first by 3 and second by 4: 12/u + 12/d = 3; 12/u + 20/d = 44/12 = 11/3. Subtracting: 8/d = 11/3 - 3 = 2/3. d = 12. Then 4/u + 4/12 = 1 -> 4/u = 2/3 -> u = 6. Speed in still water = (12+6)/2 = 9.