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Question:

anime
anime
Student

3/5 of what number = 15?

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Posted by anime on Saturday February 7, 2009 at 12:55 PM and tagged with algebra, fractions, math, maths, puzzle.

Answers:


  1. robertwilliam

    eNotes Editor

    If 3/5 is 15, then 1/5 must be 5 (15/3).

    So 5/5 of that number would be 5 x 5 = 25.

    So 25 is your answer.

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    Posted by robertwilliam on Saturday February 7, 2009 at 1:17 PM

  2. cburr
    cburr Teacher
    Middle School

    eNotes Editor

    Think about what you know.

    First, you are dealing with fractions, which means that each part has to be equal.

    Second, you know that if you add 3 of those equal parts together you get 15.

    The next step is to figure out what one of those parts would be. If you divide 15 by 3, that gives you the answer to this step.

    15/3 = 5

    We know from the problem that each of the equal parts is 1/5 of the target number. Since we know that each part = 5, then the number has to be:

    5 x 5 = 25

    *****

    The way you would write the problem out mathematically is:

    3     15

    __ = __

    5      ?

    There are two ways to solve it from here. One is that you know you have to multiply 3 by 5 to get to 15, so do the same in the denominator.

    3 x 5    15

    __    =  __

    5 x 5     25

    The other way to solve it (which you need to use if you can't figure out what to multiply by) is cross-multiplication.

    3     15

    __ = __

    5      ?

    3 x ? = 5 x 15

    3 x ? = 75

    ? = 75/3 = 25

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    Posted by cburr on Saturday February 7, 2009 at 1:18 PM


  3. kmieciakp Teacher
    High School - 12th Grade

    In word problems, "of" means multiply, "what number" means "x," and "is" means "=," so in math language, "3/5 of what number is 15" turns into: 3/5(x)=15.  Then just solve for x (invert and multiply).

    x=15(5/3)

    =75/3

    =25.

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    Posted by kmieciakp on Saturday February 7, 2009 at 6:24 PM


  4. lizbethortiz Student
    High School - 12th Grade

    (3/5)n=15   Let n equal the missing number

    you first multiply by 5 to both sides so it will cancelled out in the left and multiply 15*5 = 75

    3n=15*5

    3n=75

    Then you divide both sides by 3 so it will cancelled out in the left and leave n by itself and then just divide 75/3 = 25

    so n=25

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    Posted by lizbethortiz on Friday February 27, 2009 at 9:46 PM


  5. funkyboy Student

    This is quite simple "zero0master". All you have to do is to invert 3/5 which becomes 5/3 and multiply by 15. Your answer should be 25

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    Posted by funkyboy on Saturday June 13, 2009 at 7:29 AM