Final answer:
It will take approximately 6 years for $3,900 to double when invested at a 12% annual interest rate compounded 2 times a year.
Step-by-step explanation:
To determine how long it will take for an amount of money to double when invested at a given interest rate, we can use the rule of 72. The rule of 72 states that if you divide 72 by the interest rate, that will give you the approximate number of years it will take for your money to double. In this case, the interest rate is 12% compounded 2 times a year, so we can divide 72 by 12 to get 6. This means that it will take approximately 6 years for $3,900 to double.