Simple interest = principal * interest rate * time, where "principal" is the starting amount and "rate" is the interest rate at which the money grows per a given period of time (note: express the rate as a decimal in the formula). Time must be expressed in the same units used for time in the Rate.

An investment of d dollars at k percent simple annual interest yields $600 over a 2 year period --> $d*\frac{k}{100}*2=600$ --> $k=\frac{30,000}{d}$

What (x) dollar amount invested at the same rate will yield $2,400 over a 3 year period --> $x*\frac{k}{100}*3=2,400$ --> $x*\frac{30,000}{100d}*3=2,400$ --> $x=\frac{8d}{3}$.

Answer: E.

Or another way: In order interest to be 4 times more in 1.5 times longer time period then investment must be 4d/1.5=8d/3.

Answer: E.