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View Full Version : What am I doing wrong on this simple cardinality of unions/intersections problem?


AxiomOfChoice
Jun22-09, 04:18 PM
If I know:

|A \cup B \cup C| = 1000
|A| = 344
|B| = 572
|C| = 296
|A \cap B| = 301
|B \cap C| = 252
|A \cap C| = 213

and I use the standard formula to compute |A \cap B \cap C|, I get 554, which is absurd. Can someone tell me what's wrong here? Is there something inconsistent in the initial data we're given? If so, I can't find it...

mathman
Jun22-09, 05:55 PM
How did you get 554?

AxiomOfChoice
Jun22-09, 05:57 PM
How did you get 554?

Using the following formula:

|A \cup B \cup C| = |A| + |B| + |C| - |A \cap B| - |B \cap C| - |A \cap C| + |A \cap B \cap C|

HallsofIvy
Jun22-09, 06:14 PM
Using the following formula:

|A \cup B \cup C| = |A| + |B| + |C| - |A \cap B| - |B \cap C| - |A \cap C| + |A \cap B \cap C|

So 1000= 344+ 572+ 296- 301- 572- 296+ x.

Then you appear to have just done the arithmetic wrong. Solving this equation for x does not give anything like 554!

AxiomOfChoice
Jun22-09, 06:17 PM
So 1000= 344+ 572+ 296- 301- 572- 296+ x.

Then you appear to have just done the arithmetic wrong. Solving this equation for x does not give anything like 554!

...are you quite sure what you wrote is correct?

mathman
Jun22-09, 11:02 PM
Final analysis: The data is wrong. |A|+|B|+|C|=1212. This allows only 212 for any overlap. Since each of the pairwise intersections is more, this is impossible.

Note: minor error in the 1000= statement, the -296 should be -213, but it doesn't change the analysis.