Martin works at an aquarium shop. One day, he arrives to
work, and his boss asks him to clean a 180-gallon aquarium. He first
needs to drain the water before cleaning the tank. While draining the
water, Martin observes that after 1 hour, 150 gallons of water remain in
the tank; after 2 hours, 120 gallons remain; and after 3 hours, 90 gallons
remain in the tank.
(A)Create the function that models the number of gallons in the tank.