$(15$ Points$)A75mm$ thick square $($see below image$)$ and a cylinder with a $r=100$ mm and $h=50mm($see below image$),$ each have the same volume.

a$.$ Compare the solidification times for both the square place and the cylinder parts.

b$.$ If each is to be cast using a cylindrical riser, will each part require the same size riser to ensure proper feeding of the molten metal?

$50$mmIlongrightarrow$---100mm$