Please explain this solution EXACTLY. This is already the solution and the answer to the problem. I do not get how to get the $3931$answer$.$ It was excel solver please show step$-$by$-$step how it happened $($how to use the excel solver$).$ This is just letter a only

For reference, here is the original question: Benetton has entered into a quantity flexibility contract for a seasonal product with its retailer. If the retailer orders O units, Benetton is willing to provide up to another $35$percent if needed. Benetton$$s production cost is $$20$and they charge the retailer a wholesale price of $$36.$The retailer prices to customers at $$55$per unit. Any unsold units can be sold at a salvage value of $$25$by the retailer. Benetton can only salvage $$10$per unit for its left over inventory. The retailer forecasts demand to be normally distributed with a mean of $4,000$and a standard deviation of $1,600.$

$($a$)$How many units O should the retailer order?