Question

How many coins of radius 1 cm and thickness 0.28 CM should be melted to form a solid cylinder of height 7 cm and diameter 8 cm

Answer 1

I give a simliar qustion

find the number of coins,1.5cm in diameter and 0.2 cm thick,to be melted to form a right circular cylinder with a height of 10 cm and a diameter of 4.5cm


Given,
radius of coin = 1.5/2 cm
radius of right cylinder = 4.5/2 cm
height of coin = 0.2 cm
height of cylinder = 10 cm

...no of coins required = vol of right cylinder/vol of each coin
= pi*r*r*h/pi*r*r*h
pi gets cancelled

=r*r*h/r*r*h
=(4.5/2)2* 10/ (1.5/2)2* 0.2
= 20.25/4* 10/ 2.25/ 4* 0.2
4 gets cancelled
=20.25*10/2.25*0.2
= 202.5/0.45
=450


so, 450 coins are required