A phone company offers customers a choice of several monthly plans. Two of the simplest are plan A and plan B. Plan A has no monthly fee and charges 30 cents per minute. Plan B has a $10 monthly fee and charges 10 cents per minute. For plan A: write an equation describing the monthly cost of the plan depending of the number of minutes of using the phone. Indicate clearly what your variables represent (i.e. define the letters you use). For plan B: write an equation describing the monthly cost of this plan depending of the number of minutes of using the phone, using the same letters as for plan A.