It costs a flat fee of $3.15 to make an international call plus $0.05 for every minute that you talk. If you were charged $5.10 for making an international call, for how many minutes did you talk? minutes

y = 0.05x + 3.15 with y being the total cost and x being the number of minutes

so if u were charged 5.10.....sub in 5.10 for y
5.10 = 0.05x + 3.15
5.10 - 3.15 = 0.05x
1.95 = 0.05x
1.95 / 0.05 = x
39 = x <=== so u talked for 39 minutes