Why are free riders a problem for interest groups trying to secure collective goods?

Based on what you've learned about the professions, which ones appeal to you as a potential career and why? if none interest you as a future career, indicate another career you're interested in and explain why that career interests you.

Someone knows the answer i need in the exam

mark, a civil engineer, entered into a contract with david. as per the contract, mark agreed to design and build a house for david for a specified fee. mark provided david with an estimation of the total cost and the contract was mutually agreed upon. however, during construction, when mark increased the price due to a miscalculation on his part, david refused to pay the amount. this scenario is an example of a mistake.