100 points and !
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, 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.