Fractions Solve for Unknown X

Basic Calculator

Calculators

Math

Fractions

Fractions Solve for Unknown X

Solve for X Fraction Calculator

Enter 3 numbers and 1 unknown variable using X or any other letter

Answer:

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

Share this Answer Link: help
Paste this link in email, text or social media.

Get a Widget for this Calculator

Share this Calculator & Page

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.

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.

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/