A single deck vibrating screen with a mesh wire aperture size of $2$cm is used to screen $250$ tph of material from a cone crusher. The screen underflow material feeds an inline ball mill. It is established, through sieve analysis, that the fraction of material above $2$cm in the feed is $30\%.$ The screen underflow material flowrate was measured to be $150$ tph$.$ Calculate;

$($a$)$ The screen overall efficiency in terms of the mill feed.

$($b$)$ The $\%$ loss of undersize material in the overflow.