Question

A boat takes two hours to travel 8 km down and 8 km up the river when the water is still. How much time will the boat take to make the same trip when the river starts flowing at 4 kmph?

• A. 2 hour
• B. 2 hour 40 minute
• C. 3 hour
• D. 3 hour 40 minute

Answer 1

Answer: (A), (B)

The correct options are
A 2 hour
B 2 hour 40 minute

Speed in still water=$\frac{distance}{time}=(8+8)/2=8km/h$

Now speed during going down the stream motion will be $8+4=12km/h$ so time taken in $down$ the stream journey $t_1=\frac{distance}{speed}=8/12=2/3hour$

Speed during up the stream will be $8-4=4km/h$ so time in going down the stream, $t_2=\frac{distance}{speed}=8/4=2Hour$

Time for the trip, $t_1+t_2=2Hour40Minutes$.

Option B is correct.