Multiple choice

A stream goes downstream from one port to another in $4$ hours. It covers the same distance upstream in $5$ hours. If the speed of the stream is $2$ km/hr, the distance between the two ports is

  1. $50$ km
  2. $60$ km
  3. $70$ km
  4. $80$ km
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let boat speed be b and stream speed be s=2. Downstream: (b+2)*4 = D. Upstream: (b-2)*5 = D. 4b + 8 = 5b - 10. b = 18. D = (18+2)*4 = 80 km.

AI explanation

Let the speed of the boat in still water be x km/hr; the downstream speed is x plus 2 and the upstream speed is x minus 2. Equating the distances gives 4 times (x plus 2) equals 5 times (x minus 2), which simplifies to 4x plus 8 equals 5x minus 10, meaning x is 18 km/hr. The distance between the two ports is 4 times (18 plus 2), which equals 80 km.

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