Exam Maths Puzzle Area and Volume

A rectangle length is 5cm more than its width.if the perimeter is 40cm, find the length and width.

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Exam Other Question

6 is what percent of 24?

Please show all steps to the solution.
6(x+1) = 12(x-3)

then you get
6x+6=12x-36
My question at this point is: How do you know which side to start with. Not sure whether to start with +6 or -36 or 6x or 12x.