A firm aims to make the best decision on the inventory level to deal with the demand uncertainty in the coming season. The retail price of each sold unit is $20, the order cost of each unit is $8, and in case there is an unsold unit in inventory, the firm can still obtain a salvage value $4 for each unsold unit. Suppose the demand follows a normal distribution with mean 60 and standard deviation 12. What is the firm's best inventory level decision?