Prakash and his adventure

Prakash loves adventures. Once he came across a straight river. It was an ordinary river of width W. He wanted to make it interesting. So, he challenged himself to reach a point on other side of the river and D distance downwards from the point he stands now. He can swim at the speed of S kmph and run at the speed of R kmph. He can reach any point X on the other side of the river by swimming and t