A piano instructor charges students a one-time fee for sheet music and an hourly rate for lessons. The graph below shows the total cost for a student who has taken x lessons.
Total Cost of Piano Lessons
A graph has lessons on the x-axis, and dollars on the y-axis. Points are at (1, 50), (2, 80), (3, 110), (4, 140), (5, 170).
To increase her number of students, the instructor put a coupon in the local paper offering to waive the one-time fee for new students. What is the value of the coupon?
$14
$20
$30
$50