Math Is Fun Forum

zetafunc.

Guest

D2 Transportation Problems - North-West Corner Method

Hi,

The textbook says that in a transportation problem a degenerate solution can be avoided by not allowing diagonal moves. But does this mean that in general you're not allowed to move diagonally? They move diagonally in one of their examples but don't justify it. Is it allowed?

(Degeneracy occurs when the number of cells used in the feasible solution is (n + m - 1), where n is the number of rows and m the number of columns.)

Thanks.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: D2 Transportation Problems - North-West Corner Method

hi

I've not had to teach this so I've had a look at the Pearson textbook.  Is that the one you are using? 

http://www.pearsonschoolsandfecolleges. … apter1.pdf

because I cannot find any mention of diagonals.

If that's not the book, perhaps you could post the example.

Bobn

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

zetafunc.

Guest

Re: D2 Transportation Problems - North-West Corner Method

Just found out today it's not allowed (but he wasn't certain). I'll see if I can get a screencap of the page. It's from the new Edexcel D2 book (Keith Pledger) so I'd have to download it from somewhere...

Also, I did S1 and D1 today. Your tips really helped me relax, I just felt if I made everything really clear and showed working for everything it would be fine. Unfortunately not everything went well. I did D1 first (it was a clash with S1), and I found most of it easy apart from the last question. I won't say what it is until tomorrow, but I do feel like I could have cost myself 5 marks with the awful mistake I made! I hope that is still an A. I did S1 straight after and they said I was not allowed a break... they wouldn't let me just get up and walk around aimlessly, so I just said I needed to use the toilet (although didn't need to) so I could relax a bit for S1. The invigilators were really rude too, I felt so uncomfortable around them. Thankfully S1 was really nice, don't think I dropped any marks. [comment removed by moderator]

I don't understand why you cannot have just a little break, even if it is alone and you are accompanied by someone. I felt stressed and wanted to relax. Is there a JCQ rule about this or is it up to the centre? I am going to be having lots more of these clashes and my school was just organised terribly.

The only positive thing about doing exams at my school is that you can choose which one to do first in the event of a clash... and I found it just stupid how someone in front of me pretended to have insomnia and dyslexia so he could get 35% extra time. What a joke. (He doesn't have either of those things.) I could have used that time in the D1 exam as I finished right on the last second.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: D2 Transportation Problems - North-West Corner Method

hi zetafunc,

Glad you've found that out.  I've been teaching myself the topic and that is what I would have said.  If the number of cells is less tha m + n -1 then it may look like there's been a diagonal jump but it's just that a row and a column have reached their requirement simultaneously.

btw: I've worked in a school with Keith Pledger; he's a great teacher so you're in safe hands.

At the end of an exam you ought to be allowed a break, just to get up and walk about.  Going to the toilet seems like a good way of achieving this.

Your school want the best results from all the candidates (after all it's a "feather in their cap" isn't it?), so have a word with someone important (maths teacher / deputy head / head) and point out that candidates will do much better if they get some consideration.  It is certainly not against the rules!!!

35% extra?  That sounds unlikely.  The paperwork for getting special consideration is quite extensive and I've never known it to be more than 25%.

But what matters is your performance.  It sounds like you did ok.  I realise you count negatively; ie.  Oh dear I lost 10%; not yippie I got 90% !!!  but everyone drops a few marks somewhere.  So don't let it bother you.  Put each module aside once you've done it and concentrate on the next.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

zetafunc.

Guest

Re: D2 Transportation Problems - North-West Corner Method

But I'm not sure I got 90%. If the grade boundaries are low and it turns out I lost more marks (didn't have time to check) I could be looking at a B.

That is good to hear. I prefer his books to the older ones.

Their reason was just stupid in my opinion... you get 25% extra for dyslexia and 10% for cases like insomnia. I would have thought they'd then maybe just give 25%, rather than add the percentages. Also annoying how they didn't have any spare equipment apart from some erasers and calculators -- so the invigilator kept taking my ruler and giving it back "when I wasn't using it".

I sent an e-mail to my maths teacher regarding this, I'll see what she says. I got lucky today because the clash was an okay one... S1 + D1. I would like a break for clashes like FP2 + S4 or FP3 + M5. Those are horrible clashes. I just have to concentrate on the next exam. Chemistry and physics are next, but after that it's just maths again. M2 + D2 clash, followed by M4 (ack) the following day. Going to be a tough May and June...

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: D2 Transportation Problems - North-West Corner Method

so the invigilator kept taking my ruler and giving it back "when I wasn't using it".

Huh!  Don't they get the maths dept. to bring in a box of stuff?

Solution:  Bring a second ruler.

It is sure going to be tough!  So best wishes for a smooth ride through the rest of them.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

zetafunc.

Guest

Re: D2 Transportation Problems - North-West Corner Method

Yep, but it is a box full of barely anything. The school is quite a budget one.

I brought a second ruler but I needed it for the last question! (vertex testing, linear programming, using objective line)

Thanks, I hope it will not be too much of a bumpy ride.

By the way, if you have a question like this:

"2. bla bla bla

(a) Show that V = 3cosx
(b) Find dV/dx
(c) Find d[sup]2[/sup]V/dx[sup]2[/sup]"

Would you be allowed to do part (b) and (c) FIRST, then part (a)?

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: D2 Transportation Problems - North-West Corner Method

hi zetafunc,

I suppose you could drill a hole in your ruler and attach it by string to your wrist! 

Seems a bit extreme but it would make sure you've always got it.

They could have asked

(a) Find V in terms of x
(b) Find dV/dx
(c) Find d²V/dx²

That makes it harder for two reasons.

You don't know what V will be.
If you get it wrong, you'll get the rest of the question wrong too.

That second reason is why they avoid asking the question like that.  They like to make the parts independent as much as possible, so that you can get marks for (b) and (c) even if you cannot do (a) (or get it wrong).

Of course you'd have to get (b) to do (c) so they have to leave that dependent.

I can see why you'd like to use the clue to do those bits first, especially if you cannot see how to do (a) or you think it will take a while.

My usual remark applies here too.  Have you made it clear to the marker which answer is which?  If you always write the part numbers nice and clearly, you should achieve that.

And you can always do (b) and (c), leave (a) completely; do the rest of the paper; come back to do (a) at the end.

Just remember how they split the paper for the markers.  The sheets are scanned and a random marker will be marking question 2s for a couple of hundred candidates.  If part (a) is written at the end of question 3, he won't see it!

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

zetafunc.

Guest

Re: D2 Transportation Problems - North-West Corner Method

I suppose! Although they will probably find some way to take it from me, haha.

Well, I was asking because I don't like those potential energy questions in M4. They give you some diagram and they ask you to find the PE of the system; it's really just a geometry question and it takes me a long time to do it. In every paper I have seen they ask you to show that "V = some function of θ". I could save plenty of time by doing all the easy differentiation first. In fact, an M4 question took me 3 days to do and I'm still working on it.

So if they do say "show that..." it's because they want to make the parts independent and it's okay to do part (b) before part (a), as long as I make that clear? I won't have to write the answers in order (i.e. I wouldn't have to leave a blank space for (a) before attempting the other parts)?

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: D2 Transportation Problems - North-West Corner Method

What the markers would like is that you write, in perfect handwriting, exactly what they have on their mark scheme.  Then it's easy to give you full marks and move on.

That's what they'd like!  But it's not what they're paid for.  In practice they have to look carefully at every little bit that you write to see if there's any correct bits lurking in there.  Sometimes it isn't easy.  But the questions can be done in any order.  So do the bits that are easy for you first; don't get bogged down in a nasty bits.  Train yourself to move on when that happens and come back to them at the end. 

If you leave a space, that's handy if its big enough.  But you could end up with it even more muddly if the space you leave is too small.

Here's how I'd write it.

(2) (a) attempted after part (c).

(b) dV/dx = bla bla ...

(c) d²V/dx² = more bla bla .....

(a) .....................  working for part (a)  .........................

That seems clear to me.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

zetafunc.

Guest

Re: D2 Transportation Problems - North-West Corner Method

Okay, thanks! That might be crucial in saving my M4 grade!

I do approach exam papers differently now, thanks to your advice. Now I feel like I know exactly what to write and how to write it! Hopefully it will pay off in my remaining exams.

zetafunc.

Guest

Re: D2 Transportation Problems - North-West Corner Method

I think I can talk about the D1 paper in more detail because it's past midnight now so won't break any rules.

Here are someone's model solutions for the paper I just sat (remove spaces): sendspace.com/file/pns1te

Do you think that looks like a tougher paper than usual? I got everything the same as him apart from part (b) and for the last question I only did vertex testing on 3 points, but got the correct answer (right x and y values, right profit). I definitely lost 2 marks on part (b) because I didn't use the right method to get the answer (it was a show that question). But for the last part, do you think I'd get 1/3 or 2/3? Or possibly 3/3?

He also seems to do a bit more working than is required, or is that just me? For example, I did not put the amount of waste for each bin in the bin-packing questions...

This means I should lose a maximum of 4 marks, which I'm quite happy with!