I looked at a textbook used by a hagwon in Busan and found this question.  It was one of ten using marine invertebrate zoology to teach math.My problem is, the math seems quite challenging; well, for elementary school students (good save, Surprises!).

This answer (12) is way off, isn’t it?

Seeing this very simple calculation made me lose confidence somewhat.  I had to draw a picture before being willing to throw the given answer away and work it out.

In case anyone is interested in working it out, the answer is some distance below.  Scroll down when ready.

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I  compared the result of halving the numbers for the radius, squaring and multiplying by π.

That result being (45 squared *π)/(3.75 squared*π)= 6,361/44.175 = 144

If you’re curious, yes, I did leave in π in both locations instead of canceling them out.

Calculators do make you lazy, but I think I still know how to do some math.