Many apartment-complex owners are installing water meters for each individual apartment and billing the occupants according to the amount of water they use. this is in contrast to the former procedure of having a central meter for the entire complex and dividing up the water expense as part of the rent. where individual meters have been installed, water usage has declined 10 to 40 percent. explain this drop, referring to price and marginal utility.

The answer is; Tenants have reduced their consumption of water since individual meters have been installed, because the marginal utility of water was inferior than the price of water, so the quantity demanded decreased to an equilibrium point.

Explanation:

Consumers (in this case tenants) will consume a good (water) or service as long as its marginal utility is inferior than its price. This means they believe they are getting more than what they pay for.  

For example, the fare of a taxi ride in a big city: if consumers believe that the price of using a taxi is inferior to the benefits they get from using the taxi, then they will continue to use that service. But if the taxi's fare increases to a point where consumers believe they are being charged more than what the service actually benefits them, then they will stop using taxis.