3/5 of what number = 15?

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 !!