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Math problem

Discussion in 'DAT Discussions' started by Dencology, Jun 6, 2008.

  1. Dencology

    2+ Year Member

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    hey guys: can you do the following:

    the sum of the ages of a brother and sister ins 27 yrs. when the brother is twice as old as he is now, the sister will be three times as old as she is now. how old is the sister now?

    a. 9
    b. 12
    c. 15
    d. 18
    e. 21
     
  2. Sama951

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    I dunno I'd just jump right to the answer choices and pick one that might work, I went straight for 9 and tried it and it ended up being right lol.

    If she's 9 he has to be 18, so when he's twice that age he'll be 36 meaning she'll be 27 and 9x3=27=happy
     
  3. IdahoDoc

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    a good tip here is to just jump to the answers. But dont go to the first one! if you notice they are listed in order from smallest to largest. So start at "B" if it is too big then you know "a" is the right one (as is the case here). If it is too small then go to "D'" if it is to large then you know c is the right answer and if it is too small then you know 'e' is the right choice. So you get to compair 5 answers with two calculations.
     
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  4. Sama951

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    that's actually a fricken great tip, good thinking man
     
  5. Dencology

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    Sama it does not work. he is 18 and she is 27. their sum is not 27.
     
  6. Mstoothlady2012

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    you are adding the age after x years. The question is asking for the current age, which is 18 for brother + 9 for sister = 27
     
  7. Glycogen

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    I think the second part of the Q is to just wants to give a hint you that among the answer choices it is more logical to choose the smallest one since the Q says that her age is 3 times..... means is let than the brother so: Brother+Sister=27 and if you want to just solve for 9--> b+9=27--> b=18
    9+18=27
     
  8. Streetwolf

    Streetwolf Ultra Senior Member
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    b + s = 27
    3s = b + s (when the brother is 2x his age, he will be b years older, so the sister will also be b years older)

    Just sub in 3s = b + s into the first equation.

    3s = 27
    s = 9
    b = 18
     
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  9. TheGreenWave

    Dentist 5+ Year Member

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    This is a time-consuming question and using the answer choices is definitely the best method. Alternatively, you could setup a system of equations. The first one is obvious:
    B + S = 27

    The second is more complicated. The question tells you when B doubles, S triples. In other words, in "B" years the sisters age will be 3S. She will also be S + B years old. So the other equation is:
    S + B = 3S which simplifies to B= 2S

    When you substitue that into the first equation you get (2S) + S = 27 // S= 9

    just refreshed the page to log in and saw streetwolf posted something similar, lol, oh well...
     
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  10. DDSguyLA

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    i just plug in numbers. it works 80% of the time.
    its a. 9.
     
  11. Streetwolf

    Streetwolf Ultra Senior Member
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    Plug in numbers on the DAT if you find it to be faster but when you do these problems now, WORK THEM OUT. That way you get familiar with the mathematical concepts within the problem (in this case, system of equations) *and* you improve your english-to-math skills.
     
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  12. supraman

    supraman Boston Celtics
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    Poor written. "the sum of the ages of a brother and sister ins 27 yrs." Doesn't indicate their ages where they started or where they ended which confused the hell out of me.
    Should be something along the lines of: the sum of the ages of a brother and sister will be 27
     
  13. supraman

    supraman Boston Celtics
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    Also, the brother was 9 and the sister was 3 right? When the brother is twice his age, the sister would be three times as much as her age right? Doubling the brothers age is 18, which means he went from 9 to 18 in 9 years. So when he is twice the age, the sister is three times the age, but since he doubled in age in 9 more years, adding 3 to 9= 12? I am confused here because again, when the brother is twice the age, the sister is three times the age. Doubling the bros age is going up 9 years, so adding 9 to the current 3 to the sisters age would equal to 12, no?
     
  14. Mstoothlady2012

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    The brother is 18 and the sister is 9 now. The question says that when brother will be twice the age, the sister will be 3 times her age. Which means after 18 yrs the brother will be 18 + 18 = 36 (twice his age) and the sister will be 9 + 18 = 27 (3 times her age).
     
  15. Streetwolf

    Streetwolf Ultra Senior Member
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    The OP made a typo and it should read "brother and sister is 27 yrs".
     
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