In DS questions, you have to be very clear on the information you've been given (what do you KNOW for sure vs. what are the possibilities).
In this prompt, we have a serious lack of information. We're asked for the total value of Company H's stock, but we're given no information to start off with.
Fact 1: Investor P owns 1/4 of the shares.
This tells us nothing about the value of those shares, nor does it tell us how many people own the remaining 3/4 of the shares, so there's no way to determine the total value of the stock.
Fact 1 is INSUFFICIENT.
Fact 2: The value of Investor Q's shares of Company H's stock is $16,000.
This doesn't tell us what fraction of the total shares are owned by Investor Q, so there's no way to determine the total value of the stock.
Fact 2 is INSUFFICIENT.
Combined, we know...
Investor P owns 1/4 of the shares.
Investor Q owns an unknown fraction of the shares, which are valued at $16,000
Since we don't know if Investor Q owns 3/4 of the shares or less than 3/4 of the shares (there MIGHT be other investors), there's no way to use the value of the shares that he does own to figure out the total value of the stock.
GMAT assassins aren't born, they're made,