Why do nuclear families often have a higher living standard than extended families who live together? Why are countries with well-educated and trained citizens likely to be economically successful?

Because they are financially more secure

Explanation:

Nuclear families have more financial freedom and higher living standards than extended families who live together. Having only one child, or two at the most, guarantees that the children would grow up financially secure. There is money available to be spent to education, extra escolar activities and vacation. The living standard is higher because there are fewer people to share the available resources.

Countries with well-educated and trained citizens are likely to be economically successful because their citizen are usually employed and contributing to the countries' wealth.