Question

find the no. of coins 1.5 cm diameter and 0.2 cm thickness to be melted to form a right circular cylinder of h=10cm and diameter 4.5m

Answer 1

d=1.5 h=0.2 D=4.5x10000 H=10 Vol. Of coins=(pi)r^2×h v= pi.x0.75x0.75x0.2 Vol of cylinder=(pi)R^2xH V=pix2.25x2.25x10x10000 No.of coins= Vol of cylinder/vol of coins =2.25.2.25x10x10000/0.75x0.75x0.2 =4,50,000 Hence, 45,00,000 coins are to be melted to form the required cylinder

Answer 2

so, here's what we have got.
The coins are cylindrical, so for the coins....
r₁=1.5/2=0.75 cm
thickness is height here, so h₁=0.2 cm

Then for the cylinder..
r₂=4.5/2=2.25m=225 cm 
h₂=10 cm

so, we have got everything in cms, then all that is remained to do is to compare both ones.
so, both are cylinders.

πr₂²h₂=πr₁²h₁*n
Here the 'n' is the no. of the coins.

so, we've got 
r₂²h₂=r₁²h₁*n
(225)² (10) = (n) (0.75)² (0.2)
506250=(n) (0.5625) (0.2)
n= 506250*100000/5625*2
so, n=4,500,000 

[note: I think there may be a mistake because I'm not sure if the diameter of the main cylinder is 4.5 meters, I'd like to tell you to check out if that value is 4.5 cms.]

Hope it helped. ^_^