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Math Help - Solving for X problem?

  1. #1
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    Solving for X problem?

    Hello, I'm having trouble working with this problem:

    A capentry work can be done in 3 days by 12 persons. If 9 persons work on the same task, how long would it take them to finish it?

    The correct answer is 4, but I don't understand how to get it.
    I thought I had to do: 9 x 3 / 12, but it's not correct.

    How can I solve this?

    Thanks
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  2. #2
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    Quote Originally Posted by norifurippu View Post
    Hello, I'm having trouble working with this problem:

    A capentry work can be done in 3 days by 12 persons. If 9 persons work on the same task, how long would it take them to finish it?

    The correct answer is 4, but I don't understand how to get it.
    I thought I had to do: 9 x 3 / 12, but it's not correct.

    How can I solve this?

    Thanks
    12 persons work 3 days.
    then
    1 person works 3 * 12 days
    then
    9 persons work \dfrac{3 \cdot 12}9 days.
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  3. #3
    Like a stone-audioslave ADARSH's Avatar
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    12 persons do work in 3 days
    So
    Work done in 3days =1
    Hence work done in 1day by 12 persons = 1/3

    This is work done by 12 pewrsons
    So the work done by 4 persons = 9/(3x12) = 3/12
    thus to do complete work

    Time taken =12/3= 4 days
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  4. #4
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    Quote Originally Posted by norifurippu View Post
    Hello, I'm having trouble working with this problem:

    A capentry work can be done in 3 days by 12 persons. If 9 persons work on the same task, how long would it take them to finish it?

    The correct answer is 4, but I don't understand how to get it.
    I thought I had to do: 9 x 3 / 12, but it's not correct.

    How can I solve this?

    Thanks
    Assuming every one works at the same rate, the each is working at a rate of (1/12) job in 3 days or 1/36 job per day. 9 people will work at 9/36= 1/4 job per day and so 4/1= 4 day per job: 4 days to do the job.

    By the way: your answer, 9 x 3/12= 27/12 is slightly larger than 2 days and so less than 3 days. You are saying that 9 people can do the job faster than 12 people! That should hae told you that you should have divided where you
    multiplied.
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  5. #5
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    Talking

    Quote Originally Posted by norifurippu View Post
    A capentry work can be done in 3 days by 12 persons. If 9 persons work on the same task, how long would it take them to finish it?
    How many man-days are consumed by twelve people working three days each? (Hint: Multiply.)

    If you divide that number of man-days between nine people, how many days would that require? (Hint: Divide.)

    Have fun!
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  6. #6
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    Thank you very much everyone! at first it appeared like a very easy problem, but it ended up requiring quite an amout of thinking, thanks!
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