A cell phone company offers a contract for which the cost C, in dollars, of T minutes of telephoning is given by C=0.25(t-600)+71.95, where it is assumed the t greater than 600 minutes. What times will keep costs between $119.20 and $135.45?

$119.20 = 0.25(t-600) + 71.95

$135.45 = 0.25(t-600) + 71.95

Get rid of parenthesis and subtract 71.65 form both sides.

47.25 = .25t - 150

Add 150 to both sides.

197.25 = .25t

Multiply both sides by 4.

789 = t

Do the same for the second equation.