Final answer:
John was motivated to join the teacher's union mainly because of the 15% discount at a shopping store offered to union members.
Step-by-step explanation:
The question is asking what motivated John to join the teacher's union after 20 years of teaching without being a member. The given context suggests that the motivation for John, who has been a teacher for 20 years and has never been a member of a labor union, to chosen to join the teacher's union was primarily the 15% discount at the shopping store he frequents, offered to union members. While unions historically have provided higher wages and benefits, the decline in union membership over the decades is notable. Yet, individual incentives, such as store discounts, can still play a role in motivating someone to join a union.