CBSE Class 10 Answered

Let the speed of the boat in still water  is given to be 11kmph

 speed of the stream = y kmph

So,

 speed upstream =(11-y) kmph

 speed downstraem=(11+y)kmph

Time taken to go upstream= distance upstream / speed upstream=12/(11-y)

Time taken to go downstream = distance downstream / speed downstream=12/(11+y)

So according to question,

[12/(11-y)]+12/([11-y)]=2 hrs 45 min =11/4 hrs

Solve this and get the value of y.

We have given you the method but pl check the question again as with the values you have given the value of y is coming out to be negative.

However , the method remains the same as shown.(speed  of the stream will be given negative sign while considering the speed upstream as shown in the solution)