Question 578758
The plan:

8hours = 8(60) = 480 minutes

C= cost

C= 50 + .25(m-480)

Pay as you go:

C=.15m

.15m=.25m-70
70 = .1m
m=700

For less than 700 minutes the plan is cheaper, after that, the flat rate is cheaper

{{{ graph( 300, 300, -300, 2000, -100, 300, .25x-70, .15x) }}}