The volume of a cylinder is $v=\pi r^2 h$ where $r$ and $h$ are the radius and height, respectively. Let $n$ be the number of coins, then

$\pi (3^2)(8)=\pi (0.75^2)(0.2)(n)$

$9(8)=0.1125n$

$\dfrac{72}{0.1125}=n$

$n=640$

#.coins = V.right circular cylinder / V.coin

then (3^2 8 pi) / ((0.75)^2 * 0.2 * pi) = 640

Volume of small coin x some coins should be equal to volume of final cylinder... therefore

Pi * (radius of coin)^2 * (no. of coins) = Pi * (radius of final cylinder)^2 * (height of final cylinder)

Therefore,

Pi (0.75)^2 (0.2)*(variable 'x') = Pi * (3)^2 (8 solving for x gives

x=(9 * 8)/(0.5625*0.2)

x=640