Answer: 4.5 years
Step-by-step explanation:
You can use the Rule of 144 to calculate how long it will take for an investment to grow fourfold.
It works by dividing 144 by the interest rate the investment is growing at;
= 144/16
= 9
As the investment is being compounded semi-annually, the number of years it will take are;
= 9/2
= 4.5 years