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Mobile phone company (RO)

Two different mobile phone companies, announced the final payment for the subscription. The first one’s is about 10€ per month and it has 1000 minutes and every overtaken minute means 30 cents more to pay. The second one’s  should be paid 5€ per month for 200 included minutes and talking is billed with 0,10 Euro. a)  Which of these companies have the most efficient offer, for a needs of a person that works in in business area? b)  Define the functions that express the cost variation per number of minutes.