GMAT Problem Solving (PS) | If a square mirror has a 20-inch diagonal, what is the

I solved it using this reasoning:

When I know the diagonal I have to divide it by\(\sqrt{2}\) to get the side.
I also know that\(\sqrt{2}\) is something less than 2.

Consideration 1: 20/2 would be 10 so the perimeter would be 40
Consideration 2: If I assume that 20 is the side 20x4 gives me a perimeter of 80

So the answer must lie between 40 and 80... thus 60...
Clearly this only works because of the type of answer choices... but this is what GMAT often expects from us, right?
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