book Maths Puzzle Numerical Ability

If the perimeter of a rectangle is 40 inches, find the dimensions of the rectangle. X is the length across and x-4 down each side. Please help!

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book Other Question

(A*A)/2+(A*(30-A))=137.5
How do I solve for A?
if a = 5b
then 3/5(a) = ?

the answer is 3b but I don't understand the rationale behind it, please explain!?