To know how much of the total did Robin put into investment Y we need to know what is the amount invested in Y

let investment in x is 100x and in y 100y
so x+y=240
and intesert for y is 7y

From 1,
interset is same for X and Y but we do not have interest rate for x..not sufficient

From 2
interest from x is 5x
Amount for X or interest from x is not present..not sufficient

combining,
5x=7y or x=7y/5

7y/5 +y=240
or 12y=240*5
or y=20*5
so 7y=100/7..interest from investment Y
we can get amount invested in y..sufficient

Ans option C[spoiler][/spoiler]

quick question, is there normally an equal amount of data sufficiency problems as there are problem solving math questions on the actual GMAT?

Usually there will be 20-22 PS questions and 15-17 DS questions in actual GMAT

ok, thank you. I knew the answer to this question (and most of these DS questions) pretty much immediately, but how to solve it, I was not sure about...but that doesn't matter for these problems...so I like them

total =24,000
investment x= 24,000-x
investment y=x
interest rate of y =7/100
interest rate of x= a/100

(24000-x)(a/100)=interest from investment x
x(7/100)=interest from investment y

st 1 not sufficient
but allows for (24,000-x)(a/100)=x7/100

st 2 not sufficient
(24,000-x)5/100

combining st 1 and 2, you get (24000-x)5/100=7x/100
which x =10000