Multiple choice

Speed of motorboat in still water is $45\ km/h$. If the motorboat travels $80\ km$ along the stream in $1\ h 20\ min$, then the time taken by it to cover the same distance against the stream will be

  1. $4\ h 20\ min$
  2. $3\ h 40\ min$
  3. $2\ h 40\ min$
  4. $2\ h 55\ min$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Speed downstream = 80 / (4/3 h) = 60 km/h. Boat speed = 45 km/h, so stream speed = 15 km/h. Speed upstream = 45 - 15 = 30 km/h. Time = 80 / 30 = 8/3 hours = 2 hours 40 minutes.

AI explanation

First convert the downstream travel time of 1 hour 20 minutes into hours, giving 4/3 hours. The downstream speed equals 80 km divided by 4/3 hours, resulting in 60 km/hr. Using the downstream speed formula of boat speed plus stream speed, the stream speed must be 60 minus 45, which is 15 km/hr. The speed against the stream is therefore 45 minus 15, equaling 30 km/hr, and the time required is 80 km divided by 30 km/hr. This equals 8/3 hours, or 2 hours and 40 minutes.

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