A burglar-alarm company specializing in high-rise apartments agrees to install a central-station system in a large building for a uniform charge of $20 per apartment. The landlord guarantees a minimum purchase of 100 units. To encourage a larger number of tenants to buy, the company offers to deduct 10 cents from the $20 price for every installation in excess of 100; for example, if 110 apartments were to participate, ALL units would be priced at $19