A$1$ Auto Parts stocks standard car batteries according to the following policy: every time inventory drops to $20,$ order $30.$ The daily demand is Normally distributed with a mean of $6$ and a standard deviation of $1,$ and the lead time is $3$ days. The holding cost is $$0\mathrm{.}05$ per day.

What is the imputed cost of understocking, if unmet demand is backordered? What does this mean?

After interviewing the manager, you discover that whenever a standard battery is backordered, the salesperson will substitute a different, higher$-$quality battery for the customer, which costs the firm an extra $$6\mathrm{.}25.$ Given this information, how should A$1$ change their reorder point?