Department A can produce parts at a rate of $50/$day$.$ Department B uses those parts at a rate of $\frac{10}{?}$ day. Each day unused parts are added to inventory. At what rate does the inventory of unused parts build up$?$

The question to be answered: At what rate does inventory of unused parts build up $($i$.$e$.,$ increase$)$ per day?

The given information: Production rate $=50$ parts$/$day

Usage rate $=10$ parts $\frac{?}{?}$ day

For this simple problem, no formula or table is needed. Inventory buildup is simply the difference between the production and usage rates.

Production rate $=50$ parts$/$day

Usage rate $,=10$part$\frac{s}{d}ay$

Inventory buildup $=40$part$\frac{s}{?}$ day