If ball X is twice as big as ball Y and weighs only half as much as ball Y, then which one will have the greater density?

Density is mass divided by volume. If ball Y has mass m and volume v, then ball X has mass 0.5m and volume 2v, so its density is 0.5m ÷ 2v = 0.25(m ÷ v). Ball Y is therefore denser, so the correct choice is ball Y.

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