Answer:

Explanation:

a good but easy question. so to solve this we must have a idea what happens there at down stream and up stream . in down stream the river helps the boat to reach its destination faster by adding its speed with the boat's one ( like opportunities) . so resultant speed would be addition of the speeds of boat and stream . and at up one river tries to protest the boat by redusing its velocity ( like obstacles) . now , let us assume that speed if stream is x( an old language),. So( sped of boat is p)

( P+x)× t1=l……. 1

(P-x)×t2=l…….2

Deciding 1/2 we get ,

P+x/p-x= t2/t1…..3

By solving these we get, p=(t1+t2)x)/t2-t1. So , l=p+x. So l/p=1+x/p=1+(t2-t1)/t1+t2. This is the required time ( l/p= t3). Hope this will help. Best of luck.