Question 954052
Let the speed of streem is - a km/hr.
  
   Given - speed of a boat in still water is 15 km/hr.

Now, speed against the streem is - (15-a) km/hr.

     And with the streem is - (15+a) km/hr.

Boat goes 45 km against the streem and come back in 6 hr 45 min.

Time to take to cover 45 km against the streem is--

   
    {{{45/(15-a)}}}  hr.
.
.

And time with the streemis-

{{{45/(15+a)}}}  hr.


Hence both are equal to 6 hr 45 min or {{{6 (3/4)}}} hr or {{{27/4}}} hr.

..
..
     i.e.    
            {{{(45/(15-a)) + (45/(15+a)) = (27/4)}}}
.
.

 after simplifying this eq we can get--

.
.               a = 5 km/hr..  Ans.