An investment of $3000 was made in a certain account and earned interest that was compounded annually. The annual interest rate was fixed for the duration of the investment, and after 12 years the $3000 increased to $12000 by earning interest. In how many years after the initial investment was made the $3000 have increased to $24000 by earning interest at that rate?
Pick:  A) 16  B) 22  C) 20  D) 18  E) 30
In 12 years the investment quadrupled (from $3,000 to $12,000).   Thus, at the same rate compounded annually, it would need additional   12/2= 
<gadget id="calculator">  12/2</gadget>
<output>6</output>
6 years to double (from $12,000 to $24,000).   Therefore, 12+6=18 years are needed $3,000 to increase to $24,000.   Answer: 18.
<result>D</result>
An investment of $3000 was made in a certain account and earned interest that was compounded annually. The annual interest rate was fixed for the duration of the investment, and after 12 years the $3000 increased to $12000 by earning interest. In how many years after the initial investment was made the $3000 have increased to $24000 by earning interest at that rate?