Let us formulate two equations for both cases:
First option:
C= 40 (Cost)
Second option:
C= 15 + 3*h ( $3 is the cost of talking on the phone 60 minutes and h represents the number of hours)
Replacing the first equation in the second one, we have:
40 = 15 + 3h
40 - 15 = 3h (Subtracting 15 on both sides of the equation)
25 = 3h (Subtracting)
25/3 = h (Dividing by 3 on both sides of the equation)
8.33 =h (Dividing)
You can talk on the phone 8.33 until both plans cost the same.