An investment of $10000 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 $10000 increased to $40000 by earning interest. In how many years after the initial investment was made the $10000 have increased to $80000 by earning interest at that rate?  Pick one:  A) 16  B) 22  C) 20  D) 21  E) 18
In 12 years the investment quadrupled (from $10,000 to $40,000).   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 $40,000 to $80,000).   Therefore, 12+6=18 years are needed $10,000 to increase to $80,000.   Answer: 18.
<result>E</result>
An investment of $10000 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 $10000 increased to $40000 by earning interest. In how many years after the initial investment was made the $10000 have increased to $80000 by earning interest at that rate?