math
posted by Tom .
john has a 25 gallon fish tank. John filled his fish tank today with 24 gallons of water. the water evaporates at a rate of 1.5 gallons per week week. John adds 2 gallons to the tank every other week. At which of the following times will the tank contain 17.5 gallons?
a. exactly 11 weeks from today
b. exactly 14 weeks from today
c. exactly 9 weeks from today
d. exactly 6 weeks from today
And HOW do you get the answer??

do a series, each week.
24, 22.5, 21+2, 21.5, 20+2, 20.5, 19+2, and so on. 
Thanks for your help but I still don't get it

I think I got it....11 weeks