Explanation:
she starts with 20 pounds of rice (1 bag).
every day she uses 0.4 pounds.
to have 1.6 pounds left means she used 20-1.6 = 18.4 pounds.
so, the question really is : how many days until she used 18.4 pounds ?
that is
18.4 / 0.4 = 46
that means at the end of day 46 she will need to order a new bag.