Simple math quesiton

Discussion in 'Community Discussion' started by cantthinkofone, Jul 1, 2011.

  1. cantthinkofone macrumors 65816

    cantthinkofone

    Joined:
    Jul 25, 2004
    Location:
    Missouri, USA
    #1
    I am having a argument with my online,first mistake, math class instructor about a question on a test.

    Question is: Guy gets a 20% raise and now makes $67,200, what was is pervious salary. I said the answer is $53,760. The test said no it is $56,000.

    I used the method of 67,200 times .2 and got 13,440. Subtracted that from 67,200 and got 53,760. Or another way I could have done it was 10% of 67,200 is 6,720, times two to make it 20% and you get 13,440. 13,440 subtracted from 67,200 is 53,760.

    The method she explained I should have used and they have been using is as fallows.

    new salary = old salary + raise

    67,200 = x + 20% (x) The 20% has to be times the old salary.

    67,200 = x + .2 x Change percent to decimal

    67,200 = 1.2 x combine like terms

    67,200/1.2 = 1.2 x/1.2 Divide by 1.2 to isolate x

    $56,000 = x

    Check: 56000 + 56000(20%) = 67200

    56000 + 11200 = 67200

    So either way you get 67,200 as the new amount. Which is correct? Makes me mad that this question is so simple but is such a cluster.
     
  2. siurpeeman macrumors 603

    siurpeeman

    Joined:
    Dec 2, 2006
    Location:
    the OC
    #2
    $56,000. it's a raise of 20% of the original amount, not a raise of the new amount.
     
  3. chrfr macrumors 603

    Joined:
    Jul 11, 2009
  4. mobilehaathi macrumors G3

    mobilehaathi

    Joined:
    Aug 19, 2008
    Location:
    The Anthropocene
  5. Mac'nCheese macrumors 68030

    Mac'nCheese

    Joined:
    Feb 9, 2010
    #5
    This part of your post is your problem. You are not solving for the new amount. You are solving for the old one. You can do lots of things to the new amount and then check your math and say "well, I got 67,200 so I'm correct." You can subtract $1000 and then say "66,200 + 1000=67200, so I'm correct."
     
  6. codymac macrumors 6502

    Joined:
    Jun 12, 2009
    #6
    56k

    Put another way...
    56000 x 1.2 = 67200
    53760 x 1.2 = 64512

    Don't argue with her.
     
  7. Apple OC macrumors 68040

    Apple OC

    Joined:
    Oct 14, 2010
    Location:
    Hogtown
  8. Pink∆Floyd macrumors 68020

    Pink∆Floyd

    Joined:
    Nov 21, 2009
    Location:
    Up There
  9. -aggie- macrumors P6

    -aggie-

    Joined:
    Jun 19, 2009
    Location:
    Where bunnies are welcome.
  10. teleromeo macrumors 65816

    teleromeo

    Joined:
    Dec 2, 2006
    Location:
    kidnapped by aliens
  11. Moof1904 macrumors 65816

    Joined:
    May 20, 2004
    #11
    Frame of reference

    This illustrates one of the easiest traps to fall into with math, no matter the level at which you're studying: keep your frame of reference straight. If a person gets a 20% raise and then ends up at a given salary, the 20% raise will be a raise to the (unknown) starting point.

    In contrast, the method of subtracting 20% from the ending salary would correct if the question were 'what results from a 20% pay cut?' because that question uses the final salary as the frame of reference for the 20%.
     
  12. Gregg2 macrumors 603

    Joined:
    May 22, 2008
    Location:
    Milwaukee, WI
    #12
    You took 20% of the new salary and then subtracted that from the new salary. Since you took 20% of the higher amount, your answer is less than what the old salary was, of course. It wouldn't be much of a math problem if there was no unknown. You did your calculations with what was known. No self-respecting math instructor is going to ask "What's 2 + 2 ?"
     
  13. yg17 macrumors G5

    yg17

    Joined:
    Aug 1, 2004
    Location:
    St. Louis, MO
    #13
    Old salary is x. The raise is a 20% increase of his original salary, so the equation is x + .2x = 67000, then solve for x.

    x + .2x = 67000
    1.2x = 67000
    x = (67000/1.2)
    x = 56000
     
  14. MrSmith macrumors 68040

    MrSmith

    Joined:
    Nov 27, 2003
    #14
    I can also confirm 67,200/1.2 = 56,000 according to my calculator.
     

Share This Page