Final answer:
The rule of 72 is used to approximate the number of years it takes for an investment to double based on the interest rate. In this case, with a 4 percent interest rate, it would take 18 years for a $2000 investment to grow into $4000.
Step-by-step explanation:
The rule of 72 is a useful approximation for estimating how long it takes for an investment to double based on a given interest rate. To calculate the number of years it takes for an investment to double, divide 72 by the interest rate. In this case, the interest rate is 4 percent, so the calculation would be: 72 / 4 = 18 years. Therefore, it would take 18 years for a $2000 investment to grow into $4000 with a 4 percent interest rate.