Question

The number of coins,each of radius 0.75 cm and thickness 0.2 cm,to be melted to make a right circular cylinder of height 8 cm and base radius 3 cm is

(a) 460

(b) 500

(c) 600

(d) 640

Answer 1

Radius of each coin (r)=0.75 cm and thickness (h)=0.2 cm

$\therefore Thenvolume=\pi r^{2}h=\pi \times \frac{75}{100}\times \frac{75}{100}\times \frac{2}{10}c{m}^3$

$=\pi \times \frac{3}{4}\times \frac{3}{4}\times \frac{1}{5}=\frac{90}{80}\pi c{m}^3$

Volume of cylinder whose radius=3 cm and height 8 cm =$\pi (3{)}^3\times 8c{m}^3$

$=72\pi c{m}^3$

$\therefore No.ofcoin=\frac{72\pi \times 80}{9\pi }=640$