Hi canbtg,

This DS question essentially refers to 4 different types who were tested for an infection:

1) Tested positive AND were positive
2) Tested positive BUT were negative

3) Tested negative AND were negative
4) Tested negative BUT were positive

The question asks us what percent of the total were groups 1 and 3 (combined).

To properly answer this question, we would need LOTS of information.

Fact 1 gives us a breakdown of those who tested positive, but doesn't give us any sense of how many (or even what percent) Tested positive or negative. INSUFFICIENT.

Fact 2 gives us 90% tested negative, thus 10% tested positive, but doesn't give us any info on the "accuracy" of the tests. INSUFFICIENT

Combined, we're still missing the "breakdown" for those who tested negative, so there's no way to answer the question.

Final Answer: E

GMAT assassins aren't born, they're made,
Rich

I just want to point out that Vivek has done a perfect job employing a technique known as the Double Matrix Method. This technique can be used for most questions featuring a population in which each member has two characteristics associated with it.
Here, we have a population of patients, and the two characteristics are:
- has infection or doesn't have infection
- tested positive or tested negative

To learn more about this technique, watch our free video: https://www.gmatprepnow.com/module/gmat- ... ems?id=919

Then try these additional practice questions that can be solved using the Double Matrix Method:
- https://www.beatthegmat.com/mba/2011/05/ ... question-1
- https://www.beatthegmat.com/mba/2011/05/ ... question-2
- https://www.beatthegmat.com/mba/2011/05/ ... question-3
- https://www.beatthegmat.com/ds-quest-t187706.html
- https://www.beatthegmat.com/overlapping- ... 83320.html
- https://www.beatthegmat.com/finance-majo ... 67425.html
- https://www.beatthegmat.com/ds-french-ja ... 22297.html
- https://www.beatthegmat.com/sets-t269449.html#692540
- https://www.beatthegmat.com/in-costume-f ... tml#692116

Cheers,
Brent