1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Word Problem Credit card

Tags:
  1. Dec 17, 2004 #1
    For security, a credit card number is coded in the following way, so that it can be sent as a message. "Subtract each digit from 9"

    code the credit card number 3201 2342 3458 0931

    I dont understand the subtract each digit from 9 is that to code or uncode? Also for the above code the credit card what do I do to each number? do I add 9 or subtract 9?

    I have another credit card number that is already coded, to find the original card number do i subtract each number from 9?
     
  2. jcsd
  3. Dec 17, 2004 #2

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    I believe it's pretty clear.It gives a credit card number and askes for the coded version.It should be:6798 7657 6541 9068.
    Simple subtraction from 9999 9999 9999 9999.
    The uncoding is simple:again subtraction from that "99..."number.

    Daniel.
     
  4. Dec 17, 2004 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Does it make a difference? Try it and see.
     
  5. Dec 17, 2004 #4

    How come we subtract by 9 to uncode and code?
     
  6. Dec 17, 2004 #5

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    I guess you haven't read the post/problem very carefully.It mentions "Subtract each digit from 9".So the problem tells you how to code.As Doc depicted,it makes no difference between coding and uncoding.Both are made through the same opperation.Subtraction of each digit from 9.

    Daniel.
     
  7. Dec 17, 2004 #6
    ok my coded credit card number for 3201 2342 3458 0931 is
    6798 7657 6541 9068
    my original credit card number for another credit card 2341 0135 7923 0133 is
    7658 9864 2076 9866

    the question says find f(x) if x represents a single input digit. What is the domain of f(x)

    and find^-1(x) what is the domain of f^-1(x)?

    I dont know how to find f(x) and the inverse? What do I do? The inverse is probably all the numbers backwards but which set of numbers am I using?
     
  8. Dec 17, 2004 #7

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    The function is [itex] f:\{0,1,2,3,4,5,6,7,8,9}\rightarrow \{0,1,2,3,4,5,6,7,8,9}\ [/itex]

    [tex] f(x)=9-x. [/tex]

    Find the inverse and its domain.

    Daniel.
    What numbers backwards?????? :surprised :confused:
     
  9. Dec 17, 2004 #8
    lol sorry no numbers backwards. if f(x)=9-x then is the domain {x:x<=9,XER}?
    and is the inverse of this (x-9)/-1? If so then is the domain of the inverse {x:x>=9,XER}?
     
  10. Dec 17, 2004 #9

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    I've already told u what the function and its domain are.The inverse it's easy to find.And its domain can be seen from ISS. :tongue2:
    By "XER",do you mean:[itex] x\in R[/itex] ??If so,then it's wrong,the digits are natural numbers.

    Daniel.
     
  11. Dec 17, 2004 #10
    :tongue2: well is my inverse wrong? I dont know the notation for not real numbers :P ahhh HELP ME PLZ
     
  12. Dec 17, 2004 #11

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    As both I and Doc figured decades ago,the inverse coincides with the function.They share both the domain and the range.That is all digits from 0 till 9.
    Hope it's clear. :wink:

    Daniel.
     
  13. Dec 17, 2004 #12
    Lol thanks a lot but u said it wasnt real numbers natural numbers, what is the notation for that? Also (x-9)/-1 can I leave the inverse like that? Or can it be simplified to -(x-9)?
     
  14. Dec 17, 2004 #13

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    1.Check the posts again.I haven't used the expression "real numbers",but "natural numbers",because digits are natural numbers.
    2.[tex] f(x)=9-x ;f^{-1} (x) =9-x [/tex]
    Wasn't it obvious from what i said?????? :surprised

    Daniel.
     
  15. Dec 17, 2004 #14
    LOL WHAT AM I SAYING:P IM SAYING THAT U SAID THEY WERE NATURAL NUMBERS SOOOO IF ITS NOT XER THEN WHAT iS IT???? WHAT DO I WRITE IN THE BRACKETS {} lol UR SOOO FUNNY OMG :tongue2: and is the inverse (x-9)/-1? if so is that the same as -(x-9) :cry:

    N+?
     
    Last edited: Dec 17, 2004
  16. Dec 17, 2004 #15

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    ROFL :rofl: :rofl: :rofl: :rofl: :rofl: :rofl: :rofl:
    You're incredible..................................... :tongue2:

    I already told you what to put between those brackets.Hell,i've been trying myself,but i just couldn't... :cry: :tongue2: Only 2 '{' instead of 4 (chack post no.7,two paranthesis missing :biggrin: ).
    I already told you the inverse was the same function:9-x.It is the same with -(x-9),but why the hell write it like that??

    Daniel.
     
  17. Dec 17, 2004 #16
    LOL :rofl: :rofl: :rofl: :rofl: OMG I JUST REALIZED THAT THEY ARE THE SAME FUNCTION, ahhh lol and yes ur right y the hell would I write it like that -(x-9) and the domain is the same for both too , hhmmmmm so here this is what im gonna right now lol dont freak if this is still wrong {x:x<=9,N+} domain for f(x) and inverse? or or or {x:0,1,2,3,4,5,6,7,8,9} is that what u want me to write :tongue2: Cant u just write it out properly once :!!) plzzzzzzz lol :tongue2: DEXTERCIOBY ??? WHERE ARE U :eek: Someone please say something please.
     
    Last edited: Dec 17, 2004
  18. Dec 17, 2004 #17

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    1.I'm in my room.
    2.Though it's not in the spirit of this help forum,i'll write the answer:
    [tex] f:A\rightarrow A ,A=\{x\in N,x\leq 9[/tex]},
    [tex] f(x)=9-x [/tex]
    [tex] f^{-1}:A\rightarrow A,
    f^{-1} (x)=9-x [/tex]

    Is everything clear,FINALLLLLLLLLLLLLLLLLLLLLLLLLLLYYYYYYYYYY :tongue2: ??

    Daniel.

    PS.Anyway,i give up.It's no longer funny... :tongue2:
     
  19. Dec 18, 2004 #18

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Just one little point here...since the Naturals don't include 0, it would be better to say that x belongs to the Whole Numbers (W?).
     
  20. Dec 18, 2004 #19
    lol so XEW? instead of XEN?
     
  21. Dec 18, 2004 #20

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    You're in Canada,he's in US and i'm in Belgium.We have different views about the sets of numbers.For me,natural numbers (N) include 0.If i wanna say that zero is excluded,i write (N*).
    I was too surprised to learn that the Americans use N (natural numbers),W (whole numbers) & i can't remember what they use for {...,-3,-2,-1,0,1,...} or the negative axis.I find his weird.
    Advice:to avoid confusions and unnecessary complications simply write
    {0,1,...,9} or without the dots:{0,1,2,3,4,5,6,7,8,9}. :tongue2:

    Daniel.

    EDIT:I'd use BMW if i were u... :tongue2:
     
    Last edited: Dec 18, 2004
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Word Problem Credit card
  1. Word problem (Replies: 2)

  2. Word Problem (Replies: 1)

  3. Word problem (Replies: 6)

Loading...