3/5 of what number = 15?

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cburr's profile pic

Posted on

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

robertwilliam's profile pic

Posted on

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.

anncampbellga's profile pic

Posted on

ok - the question is 3/5 of what number = 15.

Try writing an equation to solve the problem. "of"  means to multiply. put in a variable (x) for the unknown number.

3/5 * x = 15

to get rid of a fraction that is being multiplied by a variable, you should multiply by the reciprocal of the fraction (5/3) to make the numbers cancel each other out and leave the x by itself (solve for x) - remember to multiply on both sides of the equation.

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

so x = (5*15) divided by 3

or x = 75 divided by 3

or x = 25.

So, 3/5 of 25 = 15.

middleschool-teacher's profile pic

Posted on

In this question you are solving for an the unknown (the variable).  Let's suggest using n as the variable.  When using language in math, "of" often means using multiplication.  The word "what" identifies the unknown.  If we were to substitute of what number in this equation to a math problem, then it would be

"(3/5)*n = 15" 

From here we can move the fraction (3/5) to the opposite side of the equation.  By doing so, the right side of the equation becomes 15*(5/3).  This is needed in order to isolate the variable.  The equation would then be

n = 15*(5/3)

by reducing, we solve n and n would be 

n = 25

adarshanurag's profile pic

Posted on

let the number be X

hence 3/5*x = 15

         x = 15*5/3

         x = 25

revolution's profile pic

Posted on

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





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

rnk's profile pic

Posted on

let the number be x

(3/5)x = 15

therefore x = 15 * 5/3

               = 25

funkyboy's profile pic

Posted on

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

lizbethortiz's profile pic

Posted on

(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



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

kmieciakp's profile pic

Posted on

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).




keregaba's profile pic

Posted on

Let us take unknown number be y, then 3/5 x y = 15 

3y =75 you divide by 3 each side whereby the number will be 25` `

seemaenotes's profile pic

Posted on

Let us assume 3/5 of x is 15

which means 

`3/5 * x = 15`

`3x = 15*5`

`x = (15*5)/3 `

`x = 25`


wellsn98105's profile pic

Posted on

3/5 of what number = 15

I believe that many students who struggle with these types of problems, struggle because they do not understand the language of the problem and how some words used in reading or writing may mean something very different in mathematics.

For this problem, it is very important to understand that in mathematics we use the term “of” to indicate multiplication. We also will use a “variable”, a symbol used in mathematics, to represent an unknown in a problem.

You may use any variable you would like. Let’s set up an equation by selecting a variable for our unknown. The most common symbol we use as a variable is x. Though I may choose any symbol I want I will also use x. x = what number (the unknown) Therefore,

3/5*x=15 or 3/5 x=15 Solving for x:

Step 1: Multiply both sides by 5 to clear the fraction on the left.

            5* 3/5 x=15*5      The 5’s on the left will cancel out.

=         3x = 75

Step 2: Divide both sides by 3 to solve for x.

          3x/3 = 75/3              The 3’s on the left will cancel out.

          x = 25

Therefore, 3/5 of 25 = 15 !! 

sid-sarfraz's profile pic

Posted on


3/5 of what number = 15


Let 'b' be the unknown number;

3/5 of b = 15

3/5 x b = 15

3b = 15 x 5

3b = 75

b = 75/3

b = 25

hence solved.....

malkaam's profile pic

Posted on

3/5 of what number = 15?

Let x be the unknown number,



After making the formula we need to isolate x,



x=25 Answer.

crystaltu001's profile pic

Posted on

3/5 (x) = 15

Multiply both sides by 5

3x  = 75

Divide both sides by 3

And x equals 25

udonbutterfly's profile pic

Posted on

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


ik9744's profile pic

Posted on

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.


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         

nisarg's profile pic

Posted on

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

taangerine's profile pic

Posted on

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! 

laurto's profile pic

Posted on

(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


x= 15/.6

x= 25

hope this helped!

zumba96's profile pic

Posted on

Let's put this down

3/5 of what number = 15?

`3/5x` =15


x=what number=25

acompanioninthetardis's profile pic

Posted on

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


X= 25

rimmery's profile pic

Posted on

Think of it as `3/5*x=15`

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



atyourservice's profile pic

Posted on

3/5 of what number = 15

`15x5= 75 `

75/3 = 25

so 15 is `3/5` of 25

to check


`75/5 = 15`

Wiggin42's profile pic

Posted on

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

randomtriviaman's profile pic

Posted on

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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