Final answer:
If someone who did not know about the reward returns the lost backpack, you are not legally obligated to pay the reward.
Step-by-step explanation:
In this scenario, if someone who did not know about the $20 reward returned the lost backpack, you would not be legally obligated to pay the reward. Legally, a reward is only offered to individuals who possess knowledge of the reward prior to finding and returning the lost item. Since this person did not have knowledge of the reward when they returned the backpack, they would not have a legal claim to the reward.
Learn more about Reward Posting