GMAT PREP PS question

alex.gellatly wrote:An investment of d dollars at k percent simple annual interest yields $600 interest over a 2-year period. In terms of d, what dollar amount invested at the same rate will yield $2,400 interest over a 3-year period?

(A) (2d) / 3
(B) (3d) / 4
(C) (4d) / 3
(D) (3d) / 2
(E) (8d) / 3

Thanks

Simple interest = principal * interest rate * time, where
principal = amount invested
rate = interest rate

An investment of d dollars at k percent simple annual interest yields $600 over a 2 year period implies d * k/100 * 2 = 600
k = 30,000/d

Let us assume that the amount invested at the same rate which will yield $2,400 over a 3 year period = $x

Then, x * k/100 * 3 = 2400
x * (30000/100d) * 3 = 2400
x = [spoiler]8d/3 [/spoiler]

The correct answer is E.