Using mean-per-unit estimation, an auditor has taken a sample of 200 from a population's 40,000 items; that population has a book value of $200,000. She found that in her sample the average audited value was $4.20, while the average book value was $5.20. What is the estimated total audited value of the population?

Answer is $168,000. Please show the working solution. Thank you.