In Operations Research, we build models to penalize the firm for its supply$-$demand

mismatch. In the case of excessive inventory $($i$.$e$.$ D $<500),$ the firms incurs an overage

cost of $1$ SGD per each unit of excess inventory. In the complementary case of inventory

shortage, the firms incurs an underage cost of $3$ SGD per each unit of unmet demand. The

firm$$s operational cost is the sum of its overage and underage cost.

Compute the expected operational cost incurred by the firm.