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.

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

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

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.