How many silver coins, 3.5 cm in diameter and of thickness 4 mm, must be melted to form a cuboid of dimensions 21\text{ cm}\times11\text{ cm}\times7\text{ cm}? (take \pi=\frac{22}{7})

Explanation

Volume of cuboid = 21 \times 11 \times 7 = 1617\text{ cm}^3. Volume of one cylindrical coin = \pi r^2 h = \frac{22}{7} \times (1.75)^2 \times 0.4 = 3.85\text{ cm}^3. Number of coins = \frac{1617}{3.85} = 420.

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