Question 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 of 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. The amount of water in the tank after 15 days is given by

910+(−710)+810+(−610)+⋯+310+(−110)+ 210.

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