Problema Solution
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-400)+51.95, where it is assumed that t≥400 minutes. What times will keep costs betwen 91.70 and 121.95?
Answer provided by our tutors
We need 91.70 < C < 121.95
So 91.70 < 0.25(t-400) + 51.95 < 121.95
Minus 51.95: 39.75 < 0.25(t-400) < 70
Multiply by 4: 159 < t-400 < 280
Add 400: 559 < t < 680.
Answer: 559 < t < 680