Asder Limited buys lawn sprinkler systems from a supplier at a price of $60 and sells them at a price of $100. Demand for the product is estimated to be 79 units per week. The supplier takes three weeks to supply the product from the date that Asder Limited places an order. The business holds 80 units of the product as a safety buffer at all times.

At what level of inventory (in units) should the business reorder the sprinkler system?