Water is added or drained from a tank each day. The first day, 9/10 of a gallon is added to the empty tank. The second day, 7/10 is drained from the tank. The third day, 8/10 of a gallon is added to the tank. The fourth day, 6/10 of a gallon is drained from the tank. The amount of water in the tank after 15 days is given by 9/10 + (-7/10) + 8/10 + (-6/10) + . . . + 3/10 + (-1/10) + 2/10.

How much water is in the tank after 15 days? Write your answer as a decimal.