ChatterBank0 min ago
More perms and combs
5 Answers
For some obscure reason I need to find out how many different arrangements of 4 letters I can choose from the the letters of SLEEPY SENATE.
Once again I find myself in disgreement with the book.
As I see it there are 5 cases.
4xE=1 way
3xE +1=28 ways
2xE+2xS=6 ways
All different=1680 ways
The last arrangement would be 2 alike letters plus 2 others.
I have broken this up into
2xS+1xE+1
2xS+2 from 6 (no E)
2xE+1XS+1
2xE+2 from 6 (no S)
which gives me 201 arrangements.
The book I'm using has a total of 504 arrangements for this part.
Who is right?
Once again I find myself in disgreement with the book.
As I see it there are 5 cases.
4xE=1 way
3xE +1=28 ways
2xE+2xS=6 ways
All different=1680 ways
The last arrangement would be 2 alike letters plus 2 others.
I have broken this up into
2xS+1xE+1
2xS+2 from 6 (no E)
2xE+1XS+1
2xE+2 from 6 (no S)
which gives me 201 arrangements.
The book I'm using has a total of 504 arrangements for this part.
Who is right?
Answers
Best Answer
No best answer has yet been selected by andy62. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.4E there is obviously only 1 way they could be arranged
3E + 1 The 1 could be placed in any one of the 4 positions and in each case the 3Es take the three places remaining.
But the 1 could have been chosen from any of the 7 possible letters. Hence 4 x 7 = 28 arrangements.
2E + 2different from each other. 2Es can be placed in 6 different ways (see below). There are then 7 choices for th 1st 'different' and once it has been placed there are 6 choices for the 2nd 'different' one. So 6 x 7 x 6 =252
2E + 2S It is easier to see the permutations rather than use maths logic.
EESS ESES ESSE SESE SSEE SEES So 6 cases
2S + 1different
The logic is exactly the same as for 2E + 2different above.
Hence 6 x 7 x 6 = 252
All different from each other.
8 choices for the 1st position, then 7 , then 6 , then 5.
8 x 7 x 6 x 5 = 1680
1 + 28 + 252 + 6 + 252 + 1680 = 2219
I hope this helps.
3E + 1 The 1 could be placed in any one of the 4 positions and in each case the 3Es take the three places remaining.
But the 1 could have been chosen from any of the 7 possible letters. Hence 4 x 7 = 28 arrangements.
2E + 2different from each other. 2Es can be placed in 6 different ways (see below). There are then 7 choices for th 1st 'different' and once it has been placed there are 6 choices for the 2nd 'different' one. So 6 x 7 x 6 =252
2E + 2S It is easier to see the permutations rather than use maths logic.
EESS ESES ESSE SESE SSEE SEES So 6 cases
2S + 1different
The logic is exactly the same as for 2E + 2different above.
Hence 6 x 7 x 6 = 252
All different from each other.
8 choices for the 1st position, then 7 , then 6 , then 5.
8 x 7 x 6 x 5 = 1680
1 + 28 + 252 + 6 + 252 + 1680 = 2219
I hope this helps.
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