In its first month of operations, Sandhill Company made three purchases of merchandise in the following sequence: $(1)800$ units at $ $4,(2)850$ units at $ $6,$ and $(3)950$ units at $ $7.$ Sandhill uses a periodic inventory system. $($a $1)9.$ Your answer is incorrect. Calculate weighted$-$average unit cost, $\{$Round answer to $3$ decimal places, e$.$s$.5\mathrm{.}125.)$ Weighted$-$average unit cost $