monthly service charge is flat. you have to pay it no matter how many therms you use.
since you've used only 25 therms, they'll charge you $0.6686/therm
so, total charge for using 25 therms in one month
= 8.8 + 25*0.6686=16.715+8.8 = 25.52
What is the charge for using 45 therms in one month?
45 = 25 + 20
so for first 25 therms they'll charge u .6686/therm, and rest of the 20 they'll charge u 0.85870/therm
8.8 + 25*.6686 + 20*.85870
25.52+17.174 = 42.694
