An investment of $1000 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 $1000 increased to $4000 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 correct choice from the following options
A) 16
B) 18
C) 20
D) 24
E) 30
Amount 1000 is invested for 12 yrs and yields 4000. Amount is compounded annually.   we need number of years it will take to yield 8000.   from 1000 to 4000 = 4 times   from 4000 to 8000 = 2 times.   if compounding is happening at same rate throughout, then   since, time required for 4 times = 12 yrs   =&gt;time required for 2 times = 6 yrs   Total time from initial = 12+6= 
<gadget id="calculator">12+6</gadget>
<output>18</output>
18 yrs   Ans 18.
<result>B</result>
An investment of $1000 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 $1000 increased to $4000 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?