The Data Sufficiency problem is:

A scientist is studying bacteria whose cell population doubles at constant intervals, at which times each cell in the population divides simultaneously. Four hours from now, immediately after the population doubles, the scientist will destroy the entire sample. How many cells will the population contain when the bacteria is destroyed?

(1) The population just divided and, since the population divided two hours ago, the population has quadrupled, increasing by 3,750 cells.

(2) The population will double to 40,000 cells with one hour remaining until the scientist destroys the sample.

***The problem solution stated that (2) was insufficient because the doubling period is not known. If the population just doubled to 40,000 one hour prior to the scientist destroying it, AND the scientist will destroy the population immediately after the population doubles, then the population doubles every hour and it will double to 80,000 cells prior to destruction. 80,000 was also the solution you get from (1) which is sufficient. Why is this logic wrong?

Thank you for your time,

-Colin

Colin,
Great question! Okay, let's see what we definitively know:

From the problem: Four hours from now, the population will double and then be destroyed immediately.
From Statement 2: One hour before it is destroyed (or, in other words, three hours from now), the population will double to 40,000 cells.
We need to determine the population from the sample at the time it is destroyed.

So we know:
3 hours from now--population will double to 40,000
4 hours from now--population will double again then be destroyed

It COULD be that the population doubles every hour, thus at 4 hours from now the population would be 80,000.

But could it also be that the population doubles every half hour? Sure! There is nothing that rules out that possibility. We could have:
3 hours from now--population will double to 40,000
3.5 hours from now--population will double to 80,000
4 hours from now--population will double to 160,000 and be destroyed.

So we can't obtain the exact number of bacteria before it is destroyed and Statement #2 is insufficient. Notice, the problem gives you Statement #2 after you have found 80,000 for Statement 1. Thus, you are probably thinking that 80,000 for Statement 2 is the logical result. Beware of statement carryover!

Thank you,

_________________
Jamie Nelson
ManhattanGMAT Instructor

Thanks Jamie,

I will certainly be more aware of statement carryover. It's funny how, even if you are convinced you are correct, an outside point of view can make the problem seem so much simpler. Thank you again for your assistance!

-Colin

My pleasure Colin! Yeah, statement carryover is one of the GMAT's sneakiest tricks--and one that I have struggled with too. :-) Best wishes!

_________________
Jamie Nelson
ManhattanGMAT Instructor