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
of a gallon is drained from the tank. The third day, 8/10 of a gallon is added to the tank. The fourth day, 610 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.
