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

Heyy Vips,

I tried to figure out

from 1000 to 4000 = 4 times
from 4000 to 8000 = 2 times.

But couldn't

You mean 1000 to 4000 = 4*1000
4000 to 8000 = 2 *2000

If so how u took first as 1000 and next as 2000 ..... Or any other logic?

Can you brief it plzzz... !!!

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.

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

In 12 years the investment quadrupled (from $1,000 to $4,000).

Thus, at the same rate compounded annually, it would need additional 12/2=6 years to double (from $4,000 to $8,000).

Therefore, 12+6=18 years are needed $1,000 to increase to $8,000.

Answer: B.

Hope it's clear.

I'm sorry, I'm losing you.

I'm getting

(1) (1+r)^12 = 4
(2) (1+r)^x = 8

I don't know what goes on after this... not understanding your work

But why can't we just find out what the r first and then work backwards from the solutions to find the amount of years necessary to get an interest of 7000 / total of 8000

Its actually an easier question with a simpler approach.

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
=>time required for 2 times = 6 yrs

Total time from initial = 12+6 = 18 yrs
Ans B.

Bunuel following that same logic: if I put 1000 into an account with 100% interest in year 1 I will get 2000, year 2 4000 year 3 8000. So I increased my investment eight fold in three years. Now if I want my investment to go to double to 16,000 it would take one more year. But according to what has been said above 3/4 would be the answer. Are you counting the initial deposit as year 1?

Hey Vips,

I tried to figure out

from 1000 to 4000 = 4 times
from 4000 to 8000 = 2 times.

But couldn't

You mean 1000 to 4000 = 4*1000
4000 to 8000 = 2 *2000

If so how u took first as 1000 and next as 2000 ..... Or any other logic?

Can you brief it plzzz... !!!