Good approach. Thank you

So from 1970 to 1979, the cost went up from $2,000 to $3,600 at a fixed percentage.

Letting $i$ be the fixed percentage, this can be written as:

$2000(1 + i)^9 = 3600$

Then, from 1979 to 1988, the cost went up again by the same fixed percentage. Let's call the new amount $x$. This can be written as:

$3600(1 + i)^9 = x$

Now lets put both of these equation on top of eachother:

$2000(1 + i)^9 = 3600$
$3600(1 + i)^9 = x$

We can divide the top equation by the bottom equation to get rid of that $(1 + i)^9$. If that's not a technique you're comfortable with, you can also just isolate $(1 + i)^9$, and then set up the proportion:

$\frac{2000}{3600}=\frac{3600}{x}$

Simplifying:

$\frac{5}{9}=\frac{3600}{x}$

$5x=3600*9$
$x=720*9$
$x=700*9 + 20*9$
$x=6300 + 180$
$x=6480$

And there's answer E

Its 3600/2000 not 3600/1200

since there is a percentage increase. 3600 - 2000 / 2000 * 100. = 80% change every 9 years.

Similarly getting a percent change from the year 1979 to 1988. there is a gap of 9 years. 80% of 3600 = 2880. which when added to 3600 + 2800 gives 6480.

I always follow one rule in GRE. if the questions seem complicated and are taking more than a minute to finish. You are doing it wrong