Answer is C, and what makes this one click is realizing you never need to actually find the individual prices. The ratio is enough.
Let me set up the algebra. Let n, p, f = prices of notebook, pen, folder (same for both buyers).
The question asks: what fraction of Elena's total spending went to folders?
That's 10f / (6n + 9p + 10f).
Statement (1): Elena bought 6 notebooks, 9 pens, 10 folders.
Now we know Elena's quantities, but prices are still unknown. We can write her total as 6n + 9p + 10f, but we can't simplify the fraction without knowing the price ratios. Insufficient.
Statement (2): Riley bought 18 notebooks, 27 pens, 15 folders.
We know Riley's quantities, and we know Riley spent twice as much as Elena -- but we don't know Elena's purchase breakdown. Insufficient.
Combined:
Use the constraint: Riley's total = 2 x Elena's total.
18n + 27p + 15f = 2(6n + 9p + 10f)
18n + 27p + 15f = 12n + 18p + 20f
6n + 9p = 5f
Now substitute back into Elena's total:
6n + 9p + 10f = 5f + 10f = 15f
So the fraction on folders = 10f / 15f = 2/3.
The trap I fell for the first time I saw a problem like this: I spent 90 seconds trying to figure out whether the statements give us enough to solve for individual prices. They don't -- but that's not what you need. The "Riley spent twice as much" constraint links the two sets of quantities, which pins down the price relationship, which collapses the fraction.
This is a classic DS move: you don't need absolute values, just enough structure to fix a ratio.
Answer: C