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?
Dean's height = $D$
Ron's height = $R$

$D = 4 + R$

Now we can substitute this into the original equation: 

The depth of the water is 15 times Dean's height.

$depth = 15 \times D$

$depth = 15 \times (4 + R)$

$depth = 15 \times 4 + 15 \times R$

$depth = 60 + 15 \times R$

Now we know that Ron stands at 13 feet, so: 

$13 = 60 + 15 \times R$

$-47 = 15 \times R$

$-47 / 15 = R$

$R = -3.133$

Now we can calculate Dean's height: 

$D = 4 + R$

$D = 4 - 3.133$

$D = 0.867$

So the depth of the water was: 

$depth = 15 \times D$

$depth = 15 \times 0.867$

$depth = 13$