Question

find number of coins 1.5cm of diameter and 0.2cm thick to be melted to form a right circular cylinder of height 10cm and diameter 4.5cm?​

Answer 1

Each one of those coins is also a cylinder (just a very very short one), and its volume is 

V = πr²h = π(.75)²(.2) = 9π/80 cm³ 

The right circular cylinder has volume V = π(2.25)²(10) = 405π/8 cm³. 

Divide the larger cylinder's volume by the volume of each coin = (405π/8) / (9π/80) = 450 coins