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Unknown (x) question

  1. Sep 14, 2009 #1

    How do you calculate x y?

    Lets make an example to show what I mean:
    Say there are 14 people who are buying tickets for a buss trip. In total all tickets costs 2040$. So, we got 14 people paying total 2040$. An ordinary ticket costs 160$ but there is a special price for X number of these people and their ticket will cost only 120$.

    So its 160x + 120y = 2040 and x+y=14, right?

    I don't get it.. How to I calculate this now? I'm trying to figure out how many has special price ticket. Any ideas?

  2. jcsd
  3. Sep 14, 2009 #2


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    Have you learnt any methods of solving systems of simultaneous equations?

    What happens if you multiply everything in the second equation by 120 and then subtract it from the first equation?
  4. Sep 14, 2009 #3


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    Dearly Missed

    Well, you can work a bit on the first equation:

    IF 160x+120y=2040, then obviously, we must have:

    40x+120x+120y=2040. Agreed?

    Now, since 120x+120y can be written as 120x+120y=120*(x+y), IRRESPECTIVE of the values x and y must have, we may rewrite our first equation as:

    40x+120*(x+y)=2040 (*)

    Now, can you simplify (*) further by means of your second equation, namely x+y=14?
  5. Sep 14, 2009 #4
    Wow you are really smart, didn't think about it that way!
    I got x = 9 now by doing 40x+120*14 = 2040

    it seems to be very right! thank you :)
  6. Sep 14, 2009 #5


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    Before leaving, think about the proposal of Danago.

    He said first:

    1. Multiply your second equation with 120 (ask yourself, hmm..why?)
    IF we do that we have:

    or, simplified:

    2. Now, since 120x+120y is the same number as 1680 (that's what your equation SAYS!!), clearly, we can subtract 120x+120y from the left-hand side of equation 1, whereas we subtract 1680 from the right-hand side:

    Simplifying this yields:
    which you can readily solve for x=9

    Danago's approach is more general than the one I proposed, but it seems a bit "strange" at first.
  7. Sep 14, 2009 #6
    I see.. now I'm trying with something more difficult: 98x+43y=2942 and x+y=14

    Now? I say
    x*98 + y*98 = 14*98

    43y=1570 |/43


    then we can put y into 98x+43(1570/43)=2942

    Now lets see if correct (1570/43) + 14 = 50.51162791 :/ Wrong... Am i thinking wrong now?
  8. Sep 14, 2009 #7


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    98x + 98y = 14*98 = 1372 is good. But this gives you
    (98x+43y) - (98x + 98y) = 2942 - 1372 = 1570
    (43 - 98)y = -55y = 1570

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