Multiple choice

If a person rowing at the rate of $5>$ km/h in still water, takes thrice as much time in going $40> \ km$ upstream as in going $40> \ km$ downstream, then the speed of the stream, is:

  1. $7.5\ km/hr$
  2. $2.5\ km/hr$
  3. $3.5\ km/hr$
  4. $1.5\ km /hr$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let stream speed be s. Downstream speed = 5+s, upstream = 5-s. Time = Distance/Speed. 40/(5-s) = 3 * 40/(5+s). 1/(5-s) = 3/(5+s). 5+s = 15-3s. 4s = 10. s = 2.5 km/hr.

AI explanation

Let the speed of the stream be y km/h, so the downstream speed is (5+y) and the upstream speed is (5-y). Since the time taken to go 40 km upstream is three times the time taken to go 40 km downstream, the upstream speed must be one-third of the downstream speed. This gives the equation 5-y = (5+y)/3. Multiplying by 3 gives 15-3y = 5+y, so 4y = 10. The speed of the stream is 2.5 km/hr.

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