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a) the cost is 0.06x + 19 = (6x+1900)/100 then y =(6x+1900)/100x
Answer: y = (6x+1900)/100x
b) cost = (0.05x+20.2) then average cost is (0.05x+20.2)/x
then (0.05x+20.2)/x = 0.2
then (0.05x+20.2) = 0.2x
then 0.15x=20.2
then x=20.2/0.15= 134,6666...
Answer: 135 calls
Are you sure about 20.20 and 0.05? (because solution is not an integer)