A solid sphere of radius 3 cm is melted to form a hollow cylinder of height 4 cm and external diameter 10 cm. What is the thickness of the cylinder?

Explanation

Volume of the sphere is \frac{4}{3}\pi(3^3) = 36\pi. The hollow cylinder's volume is \pi(R^2-r^2)h = \pi(5^2-r^2)4. Equating volumes gives 36 = 4(25-r^2) \implies r^2=16 \implies r=4. The thickness is R-r = 5-4 = 1 cm.

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