On average a retail store has $15$ customers in the store between $9$ and $10$ AM each day. About $50$ customers arrive between $9$ and $10$ AM each day. What is the average flow time of the customers in this store? $($Hint: Little's Law is I $=$ R x T$)$Group of answer choices$0\mathrm{.}25$ minutes$0\mathrm{.}3$ minutes$3\mathrm{.}33$ minutes$12\mathrm{.}5$ minutes$18$ minutes