the school store has1200 pendls in stock and sells and sells an average of 25 pencils per day the manager reorders when the number of pencils in stock in 650

Number of pencils in stock = 1200

Average number of pencils sold by the manager per day = 24

Number of pencils that would be sold before reordering = 1200 - 500

                                                                                        = 700

Then

The number of days after which the manager will reorder = 700/24

                                                                                            = 29.16

So the manager has to reorder after about 30 days. Since the answer comes in fraction of more than 29 days, so it has to be 30 days. I hope the procedure is clear for your understanding.

Just to be clear, this answer is not mine, but I remember seeing this question so I just copied someones' answer. Still, you might find this helpful.

Here is the original