Three of the plans offered by Virgin Mobile for cell phone service are:

Plan A 35c for each minute

Plan B $10 per month plus 10c for each minute used in the month

Plan C $20 per month plus 30c per minute for each minute

after the 200-th minute used in the month

For Plan A write an equation describing the monthly cost of the plan depending of the number of minutes used. Indicate clearly what your variables represent(i.e. define the letters you use).