Can someone please help me with a rational equation word problem?

I’m having a very difficult time getting the answer. Can someone please tell me how to set up this word problem?

‘The speed of Lazy River’s current is 5mph. If a boat travels 20 miles downstream in the same time it takes to travel 10 miles upstream, find the speed of the boat in still water.’

I keep getting 10/3 but it’s completely wrong. The answer is 15.

What am I doing wrong I put 10/X = 20/X+5 and then I cross multiply. What am I doing wrong??