Final answer:
To justify becoming a member of the zoo, you would have to ride the train 8 times, as each ride on the train with a membership costs $4 compared to $9 without a membership.
Step-by-step explanation:
To determine how many times you would have to ride the train to justify becoming a member of the zoo, we need to compare the costs with and without a membership. With a membership, each ride on the train costs $4. Without a membership, each ride on the train costs $9. The difference between these costs is $9 - $4 = $5. Therefore, you would have to ride the train 8 times to justify becoming a member of the zoo, because 8 x $5 = $40, which is the cost of the membership.