Question

A boat is able to move through still water at 20 m/s. It makes a round trip to a town 3.0 km upstream. If the river flows at 5m/s ,the time required for this round trip is:
a) 120s
b) 150s
c) 200s
d) 300s
e) 320s​

Answer 1

Answer:

Time required for round trip is 320 s.

Explanation:

Given :

Velocity of boat in still water , v = 20 m/s.

Velocity of river , V = 5 m/s.

In the given question boat make round trip to a town 3 km upstream.

Therefore, Total time = time taken in upstream + time taken in upstream.

We know , $t=\dfrac{D}{v}$   ( here all have their usual meaning ).

Total time , $t=\dfrac{3000}{(20-5)}+\dfrac{3000}{(20+5)}$    ( upstream velocity = 20-5 = 15 m/s.

                                                              downstream velocity = 20+5 = 25 m/s)

                   $t=200+120=320\ s.$

Hence, this is the required solution.

Answer 2

Answer:

Option e) 320 s

Explanation:

Here, distance = 3.0 km = 3000 m

The velocity of boat when it is going upstream;

Upstream velocity = velocity of boat in still water - velocity of river flow

So, Upstream velocity $=20m/s-5m/s=15m/s$

So,Time to go upstream

$(t_{1}) =\frac{Distance}{Velocity}=\frac{3000m}{15m/s} =200 s$

The velocity of boat when it is going downstream;

Downstream velocity = velocity of boat in still water + velocity of river flow

So, Downstream velocity $=20m/s+5m/s=25m/s$

So,Time to go downstream

$(t_{2}) =\frac{Distance}{Velocity}=\frac{3000m}{25m/s} =120 s$

So, total time (t) = $t_{1}+t_{2}=200s+120 s=320s$

Option E is the correct answer.