Another counting problem

1. Mar 5, 2014

lionely

1. The problem statement, all variables and given/known data

There are 8 seats in a railway compartment. In how many ways can 8 people be seated if 2 must have their backs to the engine and 1 must face the engine?

2. Relevant equations

3. The attempt at a solution

So 3 of them must be in those exact positions so I tried treating them like one entity so I'm ordering 6 things now, but I also need to order the 3 persons

so I have 6! * 3!, but this isn't the answer. What should I do?

2. Mar 5, 2014

nejibanana

I'm a little rusty with combinatorics, but if you ignore the stipulations, you have 8 seats. Each seat has two possible values, facing forward and facing backwards. So the total amount of combinations for that situation is 2*2*2... = 2^n = 2^8. With the stipulation, you remove 3 of the seats, as they only have one possible value now, so the total amount of combinations is 1*1*1*2*2*2..= 2^5.

3. Mar 6, 2014

lionely

i'm sorry but that's not the answer either :(

4. Mar 6, 2014

Staff: Mentor

All you need to be concerned about are the two people (of 8) who have to sit with their backs to the engine, and the one (of the remaining 6) who needs to sit facing the engine. The other five you don't need to worry about.

5. Mar 6, 2014

lionely

But isn't that what I was trying to do? :S The other 5 would just be 5! but the other three guys would 3! if i'm arranging them amongst themselves... I think...

6. Mar 6, 2014

nejibanana

I may have been thinking of permutation as opposed to combinations. Considering the permutations leads to undercounting I believe. For combinations, this page explains the situation a bit. http://en.wikipedia.org/wiki/Combination Basically it gives the formula for if you have n things and choose k of them. I think it will work for this situation, you have 8 things and you choose 3 of them.

7. Mar 6, 2014

Staff: Mentor

I don't think so. You said you were treating those three as a single entity, and this doesn't seem valid to me.

$${8}\choose{3}$$
is different from
$${{8}\choose{2}} \cdot {{6}\choose{1}}$$
The latter is a lot bigger.

8. Mar 7, 2014

pasmith

Given the constraints, you have the following possibilities:

Code (Text):

Facing   Backwards
1        7
2        6
3        5
4        4
5        3
6        2

In each case there are then two questions: How many ways to choose those who sit facing the engine, and having done so, how many ways to order those who sit in the same direction.

9. Mar 7, 2014

haruspex

I think you are supposed to assume there are four seats facing each way. Two of the people need to be seated in a particular four, and one in the other four.

10. Mar 7, 2014

Staff: Mentor

That's not an assumption I made. It's been a while since I rode on Amtrack, but my recollection is that some cars have seats that face only one direction (forward), and some, like the observation car, have seats in each of the four orientations. Also, there are lots of buses that have seat orientations other than just facing forward or backward.

11. Mar 7, 2014

lionely

So let me get this straight according to the question, the seats can only face towards to engine or the backwards to the engine?

12. Mar 8, 2014

Staff: Mentor

It's not stated in the problem (at least as far as your problem description goes). It's probably not an unreasonable assumption, but you should state it in your work.

13. Mar 8, 2014

lionely

Oh, but how would you work it without that assumption?

14. Mar 8, 2014

Staff: Mentor

Is there part of the problem you didn't include? Was there a diagram of the seat arrangement? If this is a problem your instructor wrote, you could ask for clarification on how the seats are arranged. I don't believe you can work the problem without having more information or making an assumption on the seat arrangement.

15. Mar 8, 2014

lionely

This is the exact question, it's from a book called Further Elementary Analysis by R.I. Porter, I found it on the internet. I could give you the link if you want it.

16. Mar 8, 2014

haruspex

If it's a British publication you can safely assume it's four seats forwards and four seats backwards.

17. Mar 8, 2014

pasmith

That would force there to be exactly four sitting in each orientation, which would make the given conditions on how many must sit in each orientation redundant.

18. Mar 8, 2014

haruspex

I interpret the "must"s as being personal preferences. no redundancy. Again, insistence on facing a particular way may be a peculiarly British constraint.

19. Mar 8, 2014

lionely

It is a British publication and the answer is 5760

20. Mar 8, 2014