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-500) +63.95, where it is assumed that t >= 500 minutes. what times will keep costs between $104.20 and $127.20

Answer 1

down limit is 0.2(t-500)=40.25
at t=701.25
upper limit is 0.2(t-500)=63.25
at t=816.25
sp 701.25<=t<=816.25

Answer 2

i am not sure i undeerstand

Answer 3

set C = 104.20 and solve for t:
104.2 = .025(t-500) + 63.95
40.25 = .25(t-500)
161 = t - 500
t = 661 minutes

set c = 127.20
127.2 = .25(t-500) + 63.95
63.25 = .25(t-500)
253 = t - 500
t = 753 minutes

so 661 minutes =< t =< 753 minutes