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ZIML Monthly Contest June-2022 Div M Question 10

 
 
Picture of Daniel He
ZIML Monthly Contest June-2022 Div M Question 10
by Daniel He - Thursday, 9 June 2022, 11:55 AM
 

There are nine letters, "ENDOFYEAR", but there are two letters "E". Will the correct answer be:
5!*2= 240, instead of just 5! = 120?

Thanks.

 
Picture of John Lensmire
Re: ZIML Monthly Contest June-2022 Div M Question 10
by John Lensmire - Thursday, 9 June 2022, 2:37 PM
 

The two "E"s make things a little tricky here. Note that in a rearrangement the "E"s are identical, as swapping them doesn't change anything.

Let's look at a smaller example, were we can list everything. How many different rearrangements of "ROAR" are there that contain "OAR" in the sequence.

Here there's only $2! = 2$ (not $2!\cdot 2 = 4$) different rearrangements that contain "OAR":

  1. ROAR
  2. OARR

I think it also helps to list out ALL the different rearrangements of "ROAR" in total (note there are not $4! = 24$ rearrangements, there are $4!\div 2 = 12$ of them:

  1. AORR
  2. AROR
  3. ARRO
  4. OARR
  5. ORAR
  6. ORRA
  7. RAOR
  8. RARO
  9. ROAR
  10. RORA
  11. RRAO
  12. RROA

Hope this helps!

Picture of Massimo DiLibero
Re: ZIML Monthly Contest June-2022 Div M Question 10
by Massimo DiLibero - Wednesday, 13 July 2022, 8:58 PM
 

No, since there are two E’s you divide by 2!. This is because you can switch the E’s and it will still be the same case.