Question

A person covers a certain distance downstream in 15 s and upstream in 30 s. How much time will he take to cover the same distance in still water?

Answer 1

Answer:

The correct answer is 20s.

Explanation:

Let velocity of water be u and velocity of boat in still water be v

Let distance travelled be d

We need to find d/v

t1= d/(v+u)

d= t1 v + t1 u (1)

t2= d/(v-u)

d = t2 v- t2 u (2)

From 1&2

v t1 + u t1= v t2- u t2

u(t1+ t2) = v(t2- t1)

u = v(t2-t1) / (t1+t2)

Substituting value of u in (1)

d = t1v + t1 [v(t2-t1) /t1+t2)] t1

d(t1+t2) = t1v(t1+t2) + vt1 (t2-t1)

d(t1+t2) = t1vt1 + t2t1v + vt1t2 - t1t1v

d(t1+t2) =2vt1t2

d/v = 2t1t2/( t1+t2)

So, Acc. To our ques.

2×15×30/15+30= 900/45=20s answer.

Answer 2

Answer:

30-15=15

Explanation:

15+15=30

30-10=20s

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