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? A. 16 B. 18 C. 20 D. 24 E. 30 I have the solution presented by GMAT prep but it is not the way I like to do it. My strategy would simply be to find the interest rate. And then work backwards from the solutions to get the answer (an interest of 7000, or total of 8000). But I'm having trouble calculating the rate here? Can anyone help? in 12 years he made 3000 in interest. So shouldn't it be 1000(r/100)^12 = 3000 ?? then solve for r ? I keep getting a number in the 30s, or 36.... ugh.

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An investment of $1000 was made in a certain account and ear

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Amity007

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A simple solution can be like,
if investment is taking 12 years to become 4 times, then it would take x years to become 2 time.
when we cross multiply it would be x= 12 * 2 / 4 which is 6 years.
answer would be 6 more years from 4000 to 8000. So total would be 18.

anon1

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?

A. 16
B. 18
C. 20
D. 24
E. 30

Show Spoiler

I have the solution presented by GMAT prep but it is not the way I like to do it. My strategy would simply be to find the interest rate. And then work backwards from the solutions to get the answer (an interest of 7000, or total of 8000).

But I'm having trouble calculating the rate here?

Can anyone help?

in 12 years he made 3000 in interest. So shouldn't it be 1000(r/100)^12 = 3000 ?? then solve for r ?

I keep getting a number in the 30s, or 36.... ugh.