After resting they decided to go for a swim. The depth of the water is 15 times Dean's height. Dean is 4 feet taller than Ron. If Ron stands at 13 feet how deep was the water?
Let's denote Dean's height by $d$ and Ron's height by $r$. Then we have $d=4+r$. 

The depth of the water is $15d$. 

We need to find $15d$ when $d=4+r$. 

We have $15d=15(4+r)=60+15r$. 

Therefore, the depth of the water is $60+15r$ feet.