Three basic concepts that underlie the time value of money are Inflation, Risk, and Preference. The opportunity cost rate is the rate of an alternative investment that you could invest in. Let's say that somebody takes out a car loan. The loan is for $20,000, monthly payments are made for five years, and the annual interest rate is 1.8%. 

What will the monthly payment be? Also, explain how I could double my money in one year.