IMPORTANT: USE EXCEL SOLVER AND HOW STEP$-$BY$-$STEP, TEACHER SAID 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?