Multiple choice

A boat can cover upstream 24 km in 6 hours and returns downstream 20 km in 4 hours. Then find the speed of the boat in still water (in km/hr).

  1. $4$
  2. $3.5$
  3. $5$
  4. $4.5$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let boat speed be b and stream speed be s. Upstream speed = b - s = 24/6 = 4 km/hr. Downstream speed = b + s = 20/4 = 5 km/hr. Adding: (b - s) + (b + s) = 4 + 5, so 2b = 9, giving b = 4.5 km/hr in still water. Option A (4) and C (5) are the upstream and downstream speeds respectively, which are common mistakes.

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