An investment of $100 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 $100 increased to $400 by earning interest. In how many years after the initial investment was made the $1000 have increased to $8000 by earning interest at that rate?	Choose the most appropriate option:	A) 16	B) 20	C) 18	D) 24	E) 30
In 12 years the investment quadrupled (from $100 to $400).   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 $400 to $800).   Therefore, 12+6=18 years are needed $100 to increase to $800.   Answer: 18.
<result>C</result>
An investment of $100 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 $100 increased to $400 by earning interest. In how many years after the initial investment was made the $1000 have increased to $8000 by earning interest at that rate?