Multiple choice

Two solid cylinders of radii 4 cm and 5 cm and lengths 6 cm and 4 cm respectively are recast into cylindrical disc of thickness 1 cm. The radius of the disc is

  1. 7 cm

  2. 14 cm

  3. 21 cm

  4. 28 cm

Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Volume of first cylinder: π × 4² × 6 = π × 16 × 6 = 96π. Volume of second cylinder: π × 5² × 4 = π × 25 × 4 = 100π. Total volume = 196π. When recast into a disc of thickness 1 cm: π × r² × 1 = 196π, so r² = 196, and r = 14 cm. The volume is conserved during the recasting process.