This is already the solution. I do not get how to get the $3931$ answer. It was excel solver please show step$-$by$-$step how it happened. 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?