Answer:
The total number of math folders in the classroom is 21.
Explanation:
Given: There are 7 math folder a classroom shelf which is 1/3rd of total number of math folders in the classroom.
We don't know how many math folders are there in the classroom.
Let's assume "x" be the total number of math folders in the classroom.
1/3rd of x is 7
Now let's form the equation.

Now we have to solve for x.
Multiply both sides by the reciprocal of 1/3 on both sides.
The reciprocal of 1/3 is 3.

x = 7*3
x = 21
Therefore, the total number of math folders in the classroom is 21.