Question 1006643
Let, the number of text message in a month = x

Then, the equation for total cost for a customer in a month = 10.50+0.05x

John's cost = 10.50+0.05(x)15 = 10.50+0.75=$11.25 

Daisy's cost = 10.50+0.05(x)3 = 10.50+0.15=$10.65

Judy's cost = 10.50+0.05(x)200 = 10.50+10=$20.50