A cellular phone company offers a contract for which the cost C, in dollars, of t minutes of telephoning is given by C=0.25(t-300)+79.95, where it is assumed that t≥300 minutes. What times will keep costs between $127.45 and $156.70?

Question

A cellular phone company offers a contract for which the cost C, in dollars, of t minutes of telephoning is given by C=0.25(t-300)+79.95, where it is assumed that t≥300 minutes. What times will keep costs between $127.45 and $156.70?

Answer 1

$127.45 < 0.25(t-300) + 79.95 < $156.70

127.45 < 0.25t - 75 + 79.95 < 156.70,
127.45 < 0.25t + 4.95 < 156.70,
127.45 - 4.95 < 0.25t < 156.70 - 4.95,
122.50 < 0.25t < 151.75,
490 < t < 607.

Therefore, values of t between 490 and
607 minutes will hold the cost between $127.45 and 156.70.