John street barber shop pays $25 per month for water for the first 8,000 gallons and $3.50 per thousand gallons above 8,000. calculate the total water cost when the barber shop uses 7,000 gallons, 10,000 gallons, and 13,000 gallons.

For this case, the first thing to do is find the equation of the line in order to model the problem mathematically.
 Using Excel (see attached image) we find that the equation of the line is:
 y = 0.0035x - 3

 We evaluate for:

 For x = 7000:
 y = 0.0035 * (7000) - 3
 y = 21.5 $

 For x = 10000
 y = 0.0035 * (10000) - 3
 y = 32 $

 For x = 13000
 y = 0.0035 * (13000) - 3
 y = 42.5 $

 
 The total water cost when the barber shop uses 7,000 gallons, 10,000 gallons, and 13,000 gallons is:
 21.5 $, 32 $, 42.5 $, respectively.