2008 AMC 10A Problems/Problem 23
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Problem
Two subsets of the set 
are to be chosen so that their union is 
and their intersection contains exactly two elements. In how many ways can this be done, assuming that the order in which the subsets are chosen does not matter?
Solution
First choose the two letters to be repeated in each set. 
. Now we have three remaining elements that we wish to place into two separate subsets. There are 
ways to do so (Do you see why?). Unfortunately, we have over-counted (Take for example 
and 
). Notice how 
and 
are interchangeable. A simple division by two will fix this problem. Thus we have:
This problem was discussed in an AoPS Math Jam a while back. The transcript should be located here: http://www.artofproblemsolving.com/Community/AoPS_Y_MJ_Transcripts.php?mj_id=218
See also
| 2008 AMC 10A (Problems • Resources) | ||
| Preceded by Problem 22 | Followed by Problem 24 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
