USE EXCEL SOLVER AND SHOW HOW ANSWER IS $3930$

Benetton has entered into a quantity flexibility contract with a retailer for a seasonal product. 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 it charges the retailer a wholesale price of $$36.$ The retailer prices to customers at $$55$ per unit. Any unsold units can be sold by the retailer at a salvage value of $$25.$ Benetton can salvage only $$10$ per unit for its leftover 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?

The answer is $3930\mathrm{.}93612535052$ please show me how to use EXCEL SOLVER to derive this answer $($step$-$by$-$step please$)$