A proportion is an equation that compares fractions that are equal. The proportion can be divided into 4 parts, the numerator and denominator of the two fractions.

The numerator of the first fraction and the denominator of the second fraction are called the extremes (shown in blue).
The denominator of the first fraction and the numerator of the second fraction are called the means (shown in red).

Solve for x:

1   x
- = -
2   6

Cross Product:
In order to solve for x, we use the algebraic property that the product of the extremes is equal to the product of the means. This is called the cross product.

Let’s try to solve the problem above. 

1 * 6 = 2 * x
    6 = 2x
    3 = x

If we plug in x = 3, we see that the fractions are equal.

1   3
- = -
2   6

Using what we know about cross-product, let’s try another problem!

Solve for x:

2   8
- = -
5   x

Remember that the extremes are in blue and the means are in in red.

2 * x = 8 * 5
    2x = 40
     x = 20

We plug in x = 20 and notice that the fractions are equal.

2    8
- = --
5   20