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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?

26 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 11 | 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 | Valedictorian

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 | Valedictorian

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 11 | Valedictorian

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 | TA , Grade 10 | Valedictorian

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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rimmery | Student, Undergraduate | Honors

Posted May 16, 2014 at 2:51 AM (Answer #16)

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Think of it as `3/5*x=15`

Move the fraction to the other side (by multiplying by the reciprocal)

 `x=15(5/3)`

 `x=25`

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acompanioninthetardis | Student, Undergraduate | Valedictorian

Posted May 17, 2014 at 3:41 AM (Answer #17)

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of usually is a key for multiplication so

(3/5)* X= 15

so divide both sides by 3/5

X= 15 /(3/5)

when dividing with fractions, flip the fraction and turn the division sign into a multiplication sign so

X= 15 * 5/3

15 is the same as 15/1 so just change both to a fraction and multiply

X= 15/1 * 5/3

multiply the top by top and then bottom by bottom: 15 * 5 and 3*1

X= 75/3

simplify

X= 25

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zumba96 | Student, Grade 11 | Valedictorian

Posted May 17, 2014 at 9:17 PM (Answer #18)

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Let's put this down

3/5 of what number = 15?

`3/5x` =15

(5/3)(3/5x)=15(5/3)

x=what number=25

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laurto | Student, Grade 10 | Valedictorian

Posted May 31, 2014 at 7:03 PM (Answer #19)

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(3/5)x= 15

x is the "what number" of your question

what you need to do is get the x alone. 

so divide by 3/5 or 0.6

so 

x= 15/.6

x= 25

hope this helped!

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taangerine | Student, Grade 11 | Valedictorian

Posted June 8, 2014 at 7:02 PM (Answer #20)

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Before actually solving the initial problem: 

What do we know? 3/5 of what number = 15?

  1. In a math problem, the term "of" usually stands for multiplication.
  2. And, "what number" will be a variable. 
  3. Now, that we know what "of" and "what number" represents, we can use this to solve the problem. 

Putting it all together:

  • (3/5)(x)=15

Multiply (5/3) to both sides to isolate x:

  • (5/3)(3/5)(x)=(15)(5/3)

Solve for x:

  • x=(15)(5/3)=25

Good luck! 

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nisarg | Student, Kindergarten | Valedictorian

Posted June 18, 2014 at 1:25 AM (Answer #21)

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To get the answer to this problem you have to put "n" into the equation as the number. So:

3/5(n)=15            given

3(n)=75               Multiply both sides by 5

n=25                    Divide both sides by 3

So your mystery number is 25

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ik9744 | TA , Grade 9 | Valedictorian

Posted June 19, 2014 at 11:36 PM (Answer #22)

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For this problem you can use an easy equation! You probably learn this before many time in your life. This equation follows the order PEMDAS.

Equation:

3/5(n) = 15          This is the equation given to us by you.

3(n) =15 · 5         You multiply both sides by 5.

3(n) = 75 

N = 75/3              Divide both side by 3 to leave variable by itself

N = 25         

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udonbutterfly | TA , College Freshman | Valedictorian

Posted June 30, 2014 at 3:34 AM (Answer #23)

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So we're going to take a 3/5 and multiply it times the number A which will equal 15

3/5 x A = 15

You can now  transform the equation also into 3 times A over 5 to equal 15

(3 x A)/5= 15

Now multiply the 5 on both sides

5((3 x A)/5)= (15)5

The 5 is going to be cancelled on the left so you'll get

3 x A = 75

Now divide 3 on both sides and you'll get answer

(3 x A)/3= 75/3

A=25

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crystaltu001 | Student, Grade 10 | Valedictorian

Posted July 7, 2014 at 2:54 AM (Answer #24)

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3/5 (x) = 15

Multiply both sides by 5

3x  = 75

Divide both sides by 3

And x equals 25

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nisarg | Student, Kindergarten | Valedictorian

Posted July 12, 2014 at 1:48 AM (Answer #25)

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`3/5=15/x`

`3x=75`

`x=25`

` `

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maria-vivanco | Student, Grade 11 | Valedictorian

Posted July 30, 2014 at 10:02 PM (Answer #26)

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25

if you do proportions then you would get 25 

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