Let us assume,
The speed of the of the boat =B km/hr
The speed of the stream=S km/hr
A man rows downstream at 20 km/hr.
so ,B+S=20--------------------------------------(1)
The man rows upstream at 15km/hr.
so , B-S=15 ………………………………...(2)
here they have asked the speed of the boat at still water .
Adding (1)+(2) we get ,
2B=35
=>B=17.5 km/hr
So correct answer is (A).