A cellular phone company offers a contract for which the cost C, in dollars, of t minutes of telephone is given by C=.25(t-600)+50.95, wher it is assumed that t is greater than or equal to 600 minutes. What times will keep costs between $95.45 and $135.45?

Given:

C(t)=0.25(t-600)+50.95 t≥600
We look for t such that
95.45<C(t)<135.45

Solve the first case
95.45 <C(t)
0.25(t-600)+50.95 > 95.45
0.25(t-600)>95.45-50.95=44.50
t-600 > 44.50/0.25 = 178
t > 178 + 600 = 778

Second case:
C(t)<135.45
0.25(t-600)+50.95<135.45
0.25(t-600)<84.5
t<(84.5/0.25)+600=938

Can you take it from here?