Question

If a boat goes 7 km upstream in 42 minutes and speed of the stream is 3 kmph, then the speed of the boat in still water is:

• A. 4.2 km/hr
• B. 9 km/hr
• C. 13 km/hr
• D. 21 km/hr

Answer 1

The correct option is C 13 km/hr

Rate upstream
$=(\frac{7}{42}\times 60)kmph=10kmph$

Speed of stream = 3 kmph.
Let speed in still water be x km/hr
Then speed upstream = (x - 3) km/hr.

$\therefore$ x - 3 = 10 or x = 13 kmph.