Question 890390
Which way to choose the variables is YOUR choice.  Assign them in a way that seems sensible.


The described swimming pool has a dimension of length and a dimension of width.  You need to assign a variable to each; one dimension is given as related to the other dimension.  


The description begins as {length}=15+{width}.
The perimeter is then given so that you have 2*{length}+2{width}=82.


You would not want to keep writing those words for the dimensions.  You want to assign meaningful variables to each.  Now, YOU PICK the variables.