This topic is locked. If you want to discuss this question please re-post it in the respective forum.

I've learned a few ways of solving these from the

Manhattan GMAT book but wanted someone's opinion on which way is the best or most appropriate for a certain question. Would appreciate any of your thoughts. Thanks!

Example is something like: L, M, N, O, P are sitting next to each other. O and P do not want to sit next to each other so how many total permutations are allowed.

1st way:Multiply the possibilities for each person, like a modified factorial:

P has 5 possibilities

O has 3 choices 2/5 of the time and 2 choices 3/5 of the time = 12/5 possibilities

N has 3 possibilities

M has 2 possibilities

L has 1 possibility

------------------------------

total = 5 x 12/5 x 3 x 2 x 1 = 72 possibilities

2nd way:Count number of permutations O and P will sit next to each other manually. Seats 1&2, 2&1, 2&3, 3&2, 3&4, 4&3, 4&5, 5&4 -> 8 total. The other 3 people have 3! permutations of seating arrangements still so that's 8 x 3! = 48 permutations not allowed.

5! total permutations = 120 - 48 not allowed = 72 allowed

3rd wayCount number of allowed permutations per set. I've just shifted one letter (P) down each spot, so for this one set, 3/5 of the permutations are allowed. 3/5 x 5! total permutations = 72 allowed

PLMNO - ok

LPMNO - ok

LMPNO - ok

LMNPO - not ok

LMNOP - not ok