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