Online Calculators

# Fractions Solve for Unknown X

Solve for X Fraction Calculator
Enter 3 numbers and 1 unknown variable using X or any other letter
=
x = 15

Solution by Cross Multiplication

For the equation
$\frac{5}{8} = \frac{ x}{ 24}$ The cross product is
$5 \times 24 = 8 \times x$ Solving for x
$x = \frac{5 \times 24}{8}$ and reducing
$x = 15$
Solution by Proportion

Since the equation is an equality

If
24 ÷ 8 = 3

Then it is true that
x ÷ 5 = 3

Solving for x
x = 5 × 3
x = 15

## 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.

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.

1. Cross multiply the fractions
4 * 15 = 10 * x
2. Solve the equation for x
x = (4 * 15) / 10
3. 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.

A fraction with a zero denominator is undefined.

A fraction with a zero numerator equals 0.

## 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:

$$\dfrac{a}{b} = \dfrac{c}{d}$$

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

$$\dfrac{a}{b} \times \dfrac{d}{d} = \dfrac{c}{d} \times \dfrac{b}{b}$$

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

$$\dfrac{a \times d}{b \times d} = \dfrac{b \times c}{b \times d}$$

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

$$a \times d = b \times c$$

Which is the result of cross multiplying the original equation:

$$\dfrac{a}{b} = \dfrac{c}{d}$$

### 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"; CalculatorSoup, https://www.calculatorsoup.com - Online Calculators