Question

A cellular phone company offers a contract for which cost C, in dollars, of T minutes of telephoning is given by C=0.25(t-600)+43.95 where it is assumed that T equal or less than 600 minutes. what times will keep costs between $82.70 and $130.95

Answer 1

cost always goes up with t so low time when C = 82.70 and high time when C = 130.95
so
solve
82.70=0.25(T1-600)+43.95 for T1
and solve
130.95=0.25(T2-600)+43.95 for T2
then
T1 < t < T2