Agas company has the following rate schedule for natural gas usage in single-family residences:

monthly service charge $8.80

per therm service charge
1st 25 therms $0.6686/therm
over 25 therms $0.85870/therm

what is the charge for using 25 therms in one month?
what is the charge for using 45 therms in one month?
construct a function that gives the monthly charge c for x therms of gas.

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

it takes on an electronical energy