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Thank you dundurn. I also found 2 answers using your method but I had the added complication of having the diameter of the ball in inches!!
I don't see what the right answer could be and 160 would not be to the nearest 10 of either figure as I see it. (Sorry rccatnap)
As with all these quizzes there is always one question that will be the decider as the rest are all very "google-able" so it will depend on the diameter that the setter has used for his example. As ever, his decision is final.
There is a very big prize at stake too - 200 house points - so my daughter is understandably anxious!