Multiple choice

A boat moves with a speed of $5km/h$ relative to water in a river flowing with a speed of $3 km/h$ and a width of $1km$. The minimum time taken around a round trio is

  1. $5$ minutes
  2. $20$ minutes
  3. $30$ minutes
  4. $60$ minutes
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

For the fastest crossing, the boat must aim upstream so its downstream drift is canceled. Its effective speed across the river is sqrt(5^2 - 3^2) = 4 km/h, so the round-trip time is 2/4 hour = 30 minutes.

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