Final answer:
The annual compound interest rate required to double the size of a debt in 18 years is 4%.
Step-by-step explanation:
To compute the annual compound interest rate required to double the size of a debt in 18 years, we can use the Rule of 72. The Rule of 72 is a formula that estimates how many years it will take for an investment to double at a given interest rate. To use the Rule of 72, we divide 72 by the number of years. In this case, we want to double the debt in 18 years, so we divide 72 by 18, which gives us an annual compound interest rate of 4%.