kevincan
Let's worry about the single fellow/gal S at the end. Imagine he arrives late and ensure that the couples are seperate! Again, think of two cases: ( abbrev. C1 man or wife of one couple, C2 man or wife of other couple)
(1) C1 C2 C1 C2
(2) C1 C2 C2 C1
(Notice that C1 C1 C2 C2 is unworkable)
Where could/must S sit in each case?
at teh third position...
but why have u ignored the following cases :
(3) C2 C1 C1 C2
(4) C2 C1 C1 C2
.. also.. can we not think of this problem has a "letter arrangement problem" with 3 alphabets... (C1C1)(C2C2)(thrird person)..these can be arranged in 3! ways..
Further, each of the 2 couples can be individually arranged in 2! ways..
meaning 2! for (C1male , C1female)
meaning 2! for (C2male , C2female)
.. so we have = 3! x 2! x 2!
Now, the prob. of couples siitting 2gether is = 3! x 2! x 2!/5! = 1/5
So, teh prob. of couples not sitting 2getehr is = 1- 1/5=4/5
.. however, teh answer is 2/5 ...
can you please guide me ... is my approach wrong..