Question

55. A person can swim at a speed of 9 km per hour in still
water. If the speed of the stream is 6 km per hour, then
how long does he take to swim up to a distance of 9 km
and return at the starting point?
(a) 2 hours (b) 2.5 hours
(c) 3.6 hours (d) 3.75 hours​

Answer 1

Answer:

The answer will be 3.6 hours (option c).

Step-by-step explanation:

We have given;

speed in still water = 9km/hrs;

speed of stream = 6 km/hrs;

Distance to travel = 9km;

Now, if the direction of the stream is in the same direction where the man is supposed to reach then;

Velocity of the person = 9 + 6 = 15 km/ hrs;

In his return, the velocity of stream will be against his destination, hence;

Velocity of the person in his return = 9 - 6 = 3 km/hrs;

Now, time taken to travel initial distance = 9 / 15 hours or 0.6 hours.

and time taken in his return = 9 / 3 = 3 hours.

Hence, total time taken will be;

= time taken in initial journey + time taken in return;

= 3 hours + 0.6 hours;

= 3.6 hours.

Now, if we take vice versa direction of stream then our result will still remain same because his velocity in initial journey will become -ve and in his return, his velocity will become +ve which will sum up to give same result i.e, 3.6 hours.

That's all.