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First plan: 18 + 0.14*m dollars, where m is the number of minutes.
Second plan: 10 + 0.18*m dollars, where m is the number of minutes.
For how many minutes of calls will the costs of the two plans be equal ?
18 + 0.14*m = 10 + 0.18*m ====>
18 - 10 = 0.18m - 0.14m ====> 8 = 0.04*m ====> m =
=
= 200 minutes.
Solved.