We have 3 books: one book of Mathematics BM and 2 books of Physics BP1 and
BP2. We want to display all of them ( 3 books). How many possibilities do we have?
If the two books of Physics are exactly the same, how many possibilities
do we have in this case?
The problem is the same when we want to know how many three-letters words we
can write with the letters "s", "e", and "e".
The display sets are obtained by permutations. They are
the following:
1. BM - BP1 - BP2
2. BM - BP2 - BP1
3. BP1 - BM - BP2
4. BP1 - BP2 - BM
5. BP2 - BM - BP1
6. BP2 - BM - BP1
If the two Physics books are the same, then: BP1 = BP2 = BP, hence:
1. BM - BP - BP
2. BM - BP - BP
3. BP - BM - BP
4. BP - BP - BM
5. BP - BM - BP
6. BP - BM - BP
The display sets 1 and 2 are the same. 3 and 5 are the same,
and 3 and 6 are also the same. Hence, the remaining arragement
without repetitions is:
BM - BP - BP
BP - BM - BP
BP - BP - BM
With the letters "s", "e", and "e", we have:
s e1 e2
s e2 e1
e1 s e2
e1 e2 s
e2 e1 s
e2 s e1
Then: with e1 = e2 = e, we have:
see
see
ese
ees
ees
ese
Finally, it remains the following arrangement:
see
ese
ees
Today: :
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