# Fractions Solve for Unknown X

## Calculator Use

Solve for an unknown value *x* with this fractions calculator. Find the missing fraction variable in the proportion using cross multiplication to calculate the unknown variable *x*. Solve the proportion between 2 fractions and calculate the missing fraction variable in equalities or inequalities

Enter 3 values and 1 unknown. For example, enter x/45 = 1/15. The proportion calculator solves for *x*.

## How to Solve for x in Fractions

Solve for x by cross multiplying and simplifying the equation to find x.

Example: Given the equation 4/10 = x/15 solve for x.

- Cross multiply the fractions

4 * 15 = 10 * x - Solve the equation for x

x = (4 * 15) / 10 - Simplify for x

x = 6

To check the work put the result, 6 back into the original equation

4/10 = 6/15

Cross multiply the fractions and you get

4 * 15 = 6 * 10

60 = 60

Since 60 = 60 is true, you can be sure that x = 6 is the correct answer.

## Why Does the Cross Multiplication Calculator for Fractions Work?

Cross multiplying works because you're just multiplying both sides of the equation by 1. Since multiplying anything by 1 doesn't change its value you'll have an equivalent equation.

For example, look at this equation:

If you multiply both sides by 1 using the denominators from the other side of the equation you get:

Note that this doesn't change anything, because multiplying anything by 1 doesn't change its value. So now you have:

Since the denominators are also the same here, b × d, you can remove them and say that:

Which is the result of cross multiplying the original equation:

### References

Cross Multiply from Math Is Fun at http://www.mathsisfun.com/

Cross-Multiplication from Ask Dr. Math at http://mathforum.org/

Cross Products from Ask Dr. Math at http://mathforum.org/

**Cite this content, page or calculator as:**

Furey, Edward "Fractions Solve for Unknown X"; from *http://www.calculatorsoup.com* - Online Calculator Resource.