Answer
(a) The cost would be 18 dollars.
(b) 64 minutes would've been used.
(c) The maximum minutes I could talk is 173 minutes.
(d) The implied domain is:
$[0,43200)$
Work Step by Step
(a) Calculate C(50):
$C(50)=0.26(50)+5$
$C(50)=13+5$
$C(50)=18$ dollars
(b) Swap 21.64 for C(x) and solve for x:
$21.64=0.26x+5$
$21.64-5=0.26x+5-5$
$16.64/0.26=0.26x/0.26$
$x=64$ minutes
(c) Swap 50 for C(x) and solve for x:
$50=0.26x+5$
$50-5=0.26x+5-5$
$45/0.26=0.26x/0.26$
$x\approx173$ minutes
(d) There is no such thing as talking negative minutes, so we count from zero. Theoretically, one could make a 30-day straight call, so to find the implied domain, we must convert 30 days to minutes:
$\dfrac{x}{30 \text{ days}}=\dfrac{1440 \text{ minutes}}{1\text{ day}}$
$x=43200$ minutes
Thus, the implied domain is:
$[0,43200)$
