This looks a lot more like business math than consumer math.
For all inventory balance problems, you add the input to the balance and you subtract the outflow from the balance.
11. 76 - 36 + 44 = 84
13. As with the previous months in the chart, the opening balance is the ending balance of the previous month. For March, that's February's ending balance: 210.
16. 193 + 23 + 45 - 45 - 100 = 116.
18. We know that all of the 145 received on 5/19 are included in the 150 in inventory. In addition, 5 of the units received on 5/8 also remain in inventory. The inventory value will be
... 145×$9.90 + 5×$9.75 = $1484.25
21. The reorder point is 4 + 21 = 25 days' inventory. If 750 tents represents 30 days' inventory, then the reorder point of 25 days' inventory is
... (25/30)×(750 tents) = 625 tents
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If you do what the Hint asks, you find the daily usage is (750 tents)/(30 days) = 25 tents/day. Then the safety stock is 4×25 = 100 tents, and the number of tents used during the lead time is 21×25 = 525 tents. The total of these is 100+525 = 625 tents. (Note we have done 4 calculations instead of 2, and have had to manage 2 intermediate results.) It is much simpler just to count days, IMO.
