Question

Speed of a boat in still water is 40 kmph and it takes 5 hours for it to traverse a river 300 km wide while going with the stream. What will be the difference in time for the boat between the forward and the return journey?
1. 5 hrs
2. 10 hrs
3. 15 hrs
4. 20 hrs
5. None of these

Answer 1

Correct Answer - Option 2 : 10 hrs

Given: Speed of the boat in still water = 40 kmph

Total distance = 300 km

Concept: This is a simple question of boat and stream. We need to remember that,

Going along the stream, resultant speed = speed of the boat in still water + speed of the stream

Going against the stream, resultant speed = speed of the boat in still water - speed of the stream

Formula used: s = v × t

Where s = distance, v = speed of the boat, t = time taken

Calculation:

Effective speed of the boat along the stream = 300/5 = 60 kmph

So, speed of the stream = 60 - 40 kmph = 20 kmph

While returning,

Effective speed = 40 - 20 = 20 kmph

Time required = 300/20 = 15 hrs

Required difference = 15 - 5 = 10 hrs.

Students often make a mistake of not properly reading the question and puts 15 hrs as the answer as it is also in the option.