Water is added or drained from a tank each day. The first day, 910 of a gallon is added to the empty tank. The second day, 710 o
f a gallon is drained from the tank. The third day, 810 of a gallon is added to the tank. The fourth day, 610 of a gallon is drained from the tank. how much water is in the tank after 15 days?
