Question
A hollow cylinder has an external radius of 7 cm and an
internal radius of 5 cm. If the height of the cylinder is 10 cm and it is melted to form a sphere, find the radius of the sphere.(Approx)Solution
 Volume of the hollow cylinder- Volume of cylinder = π (R2 - r²) h = π × 10 × (72 -52) = π × 10 × (49-25) = π × 10 × 24 = 240 π cm³  Volume of the sphere: Volume of sphere =4/3 πr3 Equating volumes- 240π = 4/3 πr3 240 = 4/3 r3 r3 =180 r = √180 r = 5.65 cm(approx.)
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