please do the step by step calculation. Preferably in excel

A Las Vegas supermarket bakery must decide how many wedding cakes to prepare for the upcoming weekend. Cakes cost $45 each to make, and they sell for $84 each. Unsold cakes during the weekend will be sold at 70% off during the weekdays. The weekend demand for the wedding cakes can be approximated by a normal distribution with a mean 20 and a standard deviation of 4. What would be the cost Co if the bakery over-estimates the cake demands? Input should be an exact number. 19.8