Julia owes 18.20 for the month of November. Her plan costs 9.00 for the first 600 text messages and

[math_celebrity]

Julia owes 18.20 for the month of November. Her plan costs 9.00 for the first 600 text messages and .10 cents for additional texts. How many texts did she send out?

Let m be the number of messages. We have a cost function of:
C(m) = 9 + 0.1(m - 600)

We are given C(m) = 18.20
18.20 = 9 + 0.1(m - 600)
18.20 = 9 + 0.1m - 60

Combine like terms:
18.20 = 0.1m - 51

Add 51 to each side
0.1m = 69.20

Divide each side by 0.1
m = 692