In the Sam's bookstore problem the quantity discount structure is such that all of the units ordered have the same unit cost. For example, if the order quantity is $2500,$ then each unit costs $$22\mathrm{.}25.$ Sometimes the quantity discount structure is such that the unit cost for the first so many items is one value, the unit cost for the next so many units is a slightly lower value, and so on$.$ Modify the model so that the Sam's pays $$24$ for units $1$ to $1500,$$$23$ for units $1501$ to $2500,$ and $$22$ for units $2501$ and above. For example, the total cost for an order quantity of $2750$ is $1500(24)+1000(23)+250(22).($Hint: Use IF functions, not VLOOKUP.$)$