please answer I don't undertand

Question

A cell phone company offers two monthly plans. Users of Plan A pay a monthly fee of $15, and are charged 4 cents for every minute of cell phone usage. Users of the second plan, Plan B, pay a monthly fee of $10, and are charged 5 cents for every minute of cell phone usage. Martha can afford no more than $40 per month for a cell phone plan. Which plan should she choose?

1. 
   

1. A. Plan A, because she would get up to 625 minutes of monthly cell phone usage
2. B. Plan A, because she would get up to 650 minutes of monthly cell phone usage
3. C. Plan B, because she would get up to 600 minutes of monthly cell phone usage
4. D. Plan B, because she would get up to 675 minutes of monthly cell phone usage

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Answer 1

Let x = number of minutes

Plan A costs 0.04x + 15 for x minutes

Plan B costs 0.05x + 10 for x minutes

Set each cost expression equal to 40. The solution with the larger x value is the one to choose.