Multiple choice

A boat covers a certain distance downstream in $8$ hours and comes back upstream in $10$ hours. If the speed of the current be $1$ km/hr the distance (in km) of the one way journey is

  1. $60$
  2. $70$
  3. $80$
  4. $90$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let boat speed be x, current speed be 1. Downstream speed = x+1, upstream speed = x-1. Distance D = (x+1)*8 = (x-1)*10. 8x + 8 = 10x - 10 -> 2x = 18 -> x = 9. Distance = (9+1)*8 = 80 km.

AI explanation

Let the speed of the boat in still water be x km/hr; then the downstream speed is x plus 1 and the upstream speed is x minus 1. Because the distance is constant, we equate the distances 8 times x plus 1 and 10 times x minus 1. Solving 8x plus 8 equals 10x minus 10 gives x equals 9 km/hr, and the one way distance is 8 times (9 plus 1), which is 80 km.

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