Answer
You should assign two workers per computer if you want to minimize the cost of inventorying a single item.
Work Step by Step
So
1.25 dollars per item = $\frac{(100 for computer + 25 for one worker for 1 hour)}{(100 items inventoried in 1 hour)}$
1 dollars per item = $\frac{(100 for computer + 50 for one worker for 1 hour)}{(150 items inventoried in 1 hour)}$
1.09 dollars per item = $\frac{(100 for computer + 75 for three worker for 1 hour)}{(160 items inventoried in 1 hour)}$
Hence you should assign two workers per computer if you want to minimize the cost of inventorying a single item.
