The graph of the first plan is a straight line that starts at the origin
and slopes up at the rate of 20¢/minute.
The graph of the second plan starts high up on the y-axis, at the
point where y=$50, and from there it slopes with a very gentle
slope of 2¢/minute.
The slope of the first graph is 10 times as steep as the slope of
the second graph, but they do eventually meet, at x = 278 minutes.
The second plan may seem very expensive, especially since Grace
would need to come up with $50 every month just to keep the phone
working, even before she even uses it. But if she expects to use it
more than 277 minutes in a month ... about 9 to 10 minutes each day,
then that one is actually the cheaper plan, because its minutes are
so much cheaper.
