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The square's perimeter is 12\times 4 = 48
You could also look at the area of the rectangle. This is 16 \times 9 = 144. This must be the same as the square, so the square must have side lengths of 12. Therefore its perimeter is 12 \times 4 = 48.
A rectangular field has two posts with a ring on top of each post. There are two quarrelsome goats and plenty of ropes which you can tie to their collars. How can you secure them so they can't fight each other but can reach every corner of the field?
Prove that a triangle with sides of length 5, 5 and 6 has the same area as a triangle with sides of length 5, 5 and 8. Find other pairs of non-congruent isosceles triangles which have equal areas.