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3/5 of what number = 15?

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anime | Student | eNotes Newbie

Posted February 7, 2009 at 4:55 AM via web

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3/5 of what number = 15?

Tagged with algebra, fractions, math, maths, puzzle

15 Answers | Add Yours

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cburr | Middle School Teacher | (Level 2) Associate Educator

Posted February 7, 2009 at 5:18 AM (Answer #1)

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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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robertwilliam | College Teacher | (Level 2) Senior Educator

Posted February 7, 2009 at 5:17 AM (Answer #2)

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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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kmieciakp | High School Teacher | Valedictorian

Posted February 7, 2009 at 10:24 AM (Answer #3)

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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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lizbethortiz | Student , Undergraduate | eNotes Newbie

Posted February 27, 2009 at 1:46 PM (Answer #4)

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(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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funkyboy | Student | eNotes Newbie

Posted June 12, 2009 at 11:29 PM (Answer #5)

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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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rnk | Student , Grade 10 | Honors

Posted June 12, 2010 at 3:55 PM (Answer #6)

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let the number be x

(3/5)x = 15

therefore x = 15 * 5/3

               = 25

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revolution | College Teacher | Valedictorian

Posted September 10, 2010 at 6:36 PM (Answer #7)

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Let that number be x, so the problem would look like that below:

3/5*x=15

3x/5=15

3x=15*5

3x=75

x=25 (the number). This question above is a algebraic problem.

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adarshanurag | Student , Grade 10 | Valedictorian

Posted December 7, 2013 at 5:51 AM (Answer #9)

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let the number be X

hence 3/5*x = 15

         x = 15*5/3

         x = 25

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Jyotsana | Student , Grade 10 | eNoter

Posted February 9, 2014 at 1:07 AM (Answer #11)

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3/5x=15

5/3*3/5x=15*5/3

x=25

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randomtriviaman | Student , Grade 10 | eNotes Newbie

Posted February 4, 2013 at 11:36 PM (Answer #8)

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3/5= 15, which equals 60% of x.

We can multiply 15 by 5/3 in order to get our answer.

When we do this, we end up with 25.

 

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parama9000 | Student , Grade 11 | Valedictorian

Posted January 1, 2014 at 12:54 AM (Answer #10)

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5/3x15=25

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Yojana_Thapa | Student , Grade 10 | eNoter

Posted February 22, 2014 at 10:29 PM (Answer #12)

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(3/5)x=15

(to get the x alone, divide 3/5 by both sides)

x = 15/(3/5)

x= 25

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Wiggin42 | Student , Grade 12 | eNoter

Posted March 23, 2014 at 3:58 PM (Answer #13)

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3/5 of some number equals 15. Recalling that of means multiply, we can rewrite this algebraically as: 

(3/5)x = 15

Now its a matter of isolating x. Divide both sides by 3/5

15/(3/5) = 25

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parama9000 | Student , Grade 11 | Valedictorian

Posted March 31, 2014 at 3:18 PM (Answer #14)

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25. Try working backwards.

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atyourservice | Student , Grade 10 | eNoter

Posted April 14, 2014 at 6:06 PM (Answer #15)

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3/5 of what number = 15

`15x5= 75 `

75/3 = 25

so 15 is `3/5` of 25

to check

25x3=75

`75/5 = 15`

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