Check out these overlapping sets I saw on Twitter:
In Daegu, 1900 Shincheonji Church members have been tested for coronavirus.
-1300 had symptoms & 600 did not.
-Among those 1300 with symptoms, 87.5% were confirmed with the virus.
-BUT out of the 600 WITHOUT symptoms, 70% were confirmed with coronavirus.
We can think of this scenario as:
Those WITH symptoms and those WITHOUT
Those who tested POSITIVE versus those who tested NEGATIVE
We can abbreviate as S/N and +/-
Then there are 4 possibilities:
S+ (The people who have symptoms and test positive.)
S- (The people who have symptoms and test negative. Maybe they just have a cold.)
N+ (The people who don't have symptoms but test positive. Good thing they got tested or they might have unknowingly gone around infecting people!)
N- (The people who don't have symptoms and test negative. These are the least likely to infect anyone.)
We can organize the information into a table as follows:
1300 600 1900
Observe that you can add across rows or down columns. In other words if you know 2 items in a row or column, then you also know the third. We can substituting the rest of the information in, we have:
+ 1138 420 1558
- 162 180 342
1300 600 1900
Once you have this, you know everything you need to answer any related GMAT question. They could have asked for the number who tested negative, for instance.
Hopefully this post boosted your immunity to the GMAT!