Question 1008420
First, try to make this more general using variables for all quantities.


{{{system(x=28,y=42,A=624)}}}
The x and y are pool dimensions, and A is the upper surface tile area surrounding the pool.


How wide is the uniform width border around the pool?  Call this {{{highlight_green(u)}}}.


Area of just the pool upper surface:  {{{xy}}}
Area of tile border and the pool:  {{{(x+2u)(y+2u)}}}
Area of just the tile border:  {{{(x+2u)(y+2u)-xy=A}}}
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The unknown variable, which is the question asked, is  u.
Solve for u.


Can you decide what to do with that?