Find the number of coins, 0.2 cm thick and 2 cm in diameter, to be melted to form a right circular cylinder of height 12 cm and diameter 5 cm.

Correct option is C

Given:

Thickness of coins = 0.2cm

Diameter of coins = 2cm

Height of cylinder =12cm

Diameter of cylinder = 5cm

Concept Used: 

When coins are melted and carved into a cylinder the volume remains same.

Coin is also a form of cylinder where diameter is greater than height

Formula Used:

Volume of cylinder = $$\pi \times (r^2) \times h$$ 

where r = radius, h = height

Solution:

Volume of cylinder = $$n \times$$​ Volume of coins

Radius of coins = $$\frac{2}{2} =1 \ cm$$​​

Radius of cylinder = $$\frac{5}{2}\ cm$$​​

​$$\pi \times \left(\frac{5}{2}\right)^2 \times 12 = n\times \pi \times (1)^2 \times (0.2)$$​​

​$$n = \frac{25\times 12}{4 \times (0.2)}$$​​

n = 375

Thus,  number of coins required are 375