1. avatar says

    Hi sir, I was working a question from my BPP kit on linear programming and there were products A and B. I did the graph but only product A had a demand of 1000 limit. But on my graph I named product B as my X-axis and product A as my Y-axis. Then when I plotted the Demand line, it was a horizontal one instead of vertical(as I used y-axis for A). In the kit, it’s done the opposite way i.e A being x-axis. My question is, is it a fault in the exams?

    Thx :)

  2. avatar says

    I really enjoyed this lecture. Thanks a lot Mr Moffat. I have a question though: Does it matter if Standard or Executive (as in the example) is plotted on the Y axis? Can Executive be on the Y axis also? Thank you.

    • Avatar of johnmoffat says

      Several reasons!

      You can calculate where each 2 lines cross, but if there are more than 2 constraints it will not be obvious without the graph which points are not feasible – I mention this in the lecture.

      Also, if the question later mentions that one of the constraints changes, then the effect is more obvious if you have the graph.

      Finally, the examiner will ask for the graph and therefore if you have not drawn it you will lose marks!

    • Avatar of johnmoffat says

      I think you mean the contribution at each corner of the feasible region (because the lines themselves do not have contributions!).

      Assuming that is what you mean, then you can do this and then choose the one with the biggest contribution, However, you must write what you are doing (so the marker is clear) – something like ‘because of the theory of iso-contribution line, I have checked all the corners and the best one is…….’

      You must still graph the constraints because it will be asked for, and without it you will not know what the feasible region is.
      Also, read the question carefully – if it actually asks for the iso-contribution line then obviously you must draw it.

  3. avatar says

    Hi Mr. John, I have a doubt in iso contribution area. That whether it is compulsory that our iso contribution line to be remain in the feasible region area or contribution line can be drawn pass the feasible region.

    Can you please clear my doubt.


    • Avatar of johnmoffat says

      The iso-profit line is best drawn within the feasible area. (It can be outside, but it makes it easier to make mistakes and choose the wrong point. Also it means that your constraints will not be filling the graph as much as they could have)

  4. avatar says

    Thank you Sir John for this. But during the lecture you said you’ll explain later on how the values were derived for the X-AXIS AND Y-AXIS, giving a maximum value of 40. However, the lecture ended but no explanation was given as to how we derive those values? Could you please explain now.

    • Avatar of johnmoffat says

      It does not matter what maximum values the axes go up to.

      However to make sure that I get a nice big graph, what I do is decide on the points I want to plot for each of the constraints first. Then I fix the scales on the axes to make sure that they are at (or a little above) the maximum I want to plot for x and the maximum I want to plot for y.

      This then makes sure that the constraints will actually fit on the graph, and also that they are nice and big :-)

  5. avatar says

    Thank you very much! i appeared for my F2 exam in november 2011, my first attempt of F5 will be dis june, dis was a great revision lecture! now i ‘ll move on to the second lecture! Thank u very much sir!

    • avatar says

      at point A, contribution is $ 216, At point C; contribution is $ 210 and at point B; contribution is $ 225. Thats why we picked point B, because at that point the contribution is the highest (max.) – plz correct me if iam mistaken.

      • Avatar of johnmoffat says

        You can check all the corners by all means, however if you draw the iso-contribution line then you can find out from that which is the best corner and then that is all you need to check.

        (Checking all the corners is fine, but you must make sure you understand about the contribution line because you just could be asked about it specifically)

    • Avatar of johnmoffat says

      Linear programming will certainly come again soon. I think there is a reasonable chance it will be this June, but that is only a guess – you had better make sure you are happy with everything else as well :-)

      • Avatar of johnmoffat says

        If it does come up in June then it could be either. In the past you have usually been required to draw the graph yourself – the one in 2010 was unusual in that it gave you the graph.

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