In its first month of operations, Waterway Company made three purchases of merchandise in the following sequence: $(1)320$ units at

$$8.(2)440$ units at $$9,$ and $(3)240$ units at $$10.$

$($a$)$

Calculate the average unit cost. $($Round answer to $2$ decimal places, e$.$g$.15\mathrm{.}25.)$

Average unit cost

$