Water is added or drained from a tank each day. the first day, 1310 of a gallon is added to the empty tank. the second day, 1110 of a gallon is drained from the tank. on the third day, 1210 of a gallon is added to the tank. on the fourth day, 1 gallon is drained from the tank. the amount of water in the tank after 15 days is given by

1310+(−1110)+1210+(−1010)+⋯+710+(−5 10)+610.

how much water is in the tank after 15 days?