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Cross Multiply

Jo-ann Caballes

Updated on January 8, 2024

Welcome to Brighterly, where we make math fun and engaging for children! Today, we will be exploring the concept of cross multiplication, a powerful technique that simplifies the process of comparing and solving fractions and ratios. At Brighterly, we believe that grasping essential math concepts like cross multiplication will empower children to excel in their math journey and tackle more complex problems with ease.

Cross multiplication is a valuable skill that helps children understand the relationships between fractions and ratios, making it easier for them to solve problems and build a strong foundation in mathematics. By incorporating cross multiplication into their math toolbox, kids can become more confident and efficient in their problem-solving abilities.

What Is Cross Multiplication?

Cross multiplication is a mathematical technique used to compare or simplify fractions and ratios. This method involves multiplying the numerator of one fraction by the denominator of the other and vice versa, resulting in two products that can be compared or used to find an unknown value. Cross multiplication is a useful tool for children as they learn the fundamentals of fractions and ratios in mathematics. Mastering cross multiplication early on allows kids to tackle more complex problems with confidence.

While cross multiplication can seem intimidating at first, with consistent practice and a solid understanding of the concept, children can quickly grasp its utility. It is important to remember that cross multiplication is only applicable to fractions and ratios and should not be confused with other mathematical operations.

How to Cross Multiply?

Cross multiplication is a straightforward process, but it is essential to follow the correct steps to ensure accurate results. The process involves performing basic multiplication and comparison operations, which children should already be familiar with. Here is an overview of how to cross multiply:

1. Identify the fractions or ratios you want to compare or simplify.
2. Write down the fractions or ratios side by side, ensuring that the numerators are on the top and the denominators are on the bottom.
3. Multiply the numerator of the first fraction by the denominator of the second fraction and record the result.
4. Multiply the denominator of the first fraction by the numerator of the second fraction and record the result.
5. Compare the two products obtained in steps 3 and 4 to determine if the fractions are equivalent, larger, or smaller.

Steps to Cross Multiply

Here are the detailed steps to cross multiply fractions or ratios:

1. Write the two fractions side by side, with an equal sign between them if you’re comparing them for equivalence or an inequality sign if you’re comparing their sizes.
2. Draw a diagonal line from the top-left corner (numerator) of the first fraction to the bottom-right corner (denominator) of the second fraction.
3. Draw another diagonal line from the bottom-left corner (denominator) of the first fraction to the top-right corner (numerator) of the second fraction.
4. Multiply the numbers connected by each diagonal line and write the product on the corresponding side.
5. Compare the two products to determine the relationship between the fractions or solve for the unknown variable if one is present.

Comparing Fractions by Cross Multiply

Cross multiplication is a valuable tool for comparing fractions without having to find a common denominator. By multiplying the numerators and denominators diagonally, you can quickly determine if the fractions are equal, larger, or smaller. Here’s an example:

1. Write the fractions side by side: 3/4 and 6/8.
2. Multiply the top-left (3) by the bottom-right (8) and write the product (24) on the left side of an inequality sign.
3. Multiply the bottom-left (4) by the top-right (6) and write the product (24) on the right side of the inequality sign.
4. Compare the two products: 24 = 24. Since the products are equal, the fractions are equivalent.

Comparing Ratios by Cross Multiply

Cross multiplication can also be used to compare ratios, which are similar to fractions. The process is essentially the same as with fractions, but with slightly different notation. Here’s an example:

1. Write the ratios side by side: 4:5 and 12:15.
2. Convert the ratios to fractions: 4/5 and 12/15.
3. Follow the same steps as with fractions to cross multiply and compare the ratios.

Cross Multiply With One Variable

Cross multiplication can be used to solve equations with one variable involving fractions. Here’s an example:

1. Write the equation: 2/3 = x/6.
2. Multiply the top-left (2) by the bottom-right (6) and write the product (12) on the left side of the equal sign.
3. Multiply the bottom-left (3) by the top-right (x) and write the product (3x) on the right side of the equal sign.
4. The equation now looks like this: 12 = 3x.
5. Solve for x: x = 4.

Cross Multiply With Two Variables

Cross multiplication can also be used to solve equations with two variables involving fractions. Here’s an example:

1. Write the equation: a/b = c/d.
2. Multiply the top-left (a) by the bottom-right (d) and write the product (ad) on the left side of the equal sign.
3. Multiply the bottom-left (b) by the top-right (c) and write the product (bc) on the right side of the equal sign.
4. The equation now looks like this: ad = bc.

Depending on the information given, you may need to use additional methods to solve for both variables.

Cross Multiply with Variables on Both Sides

In some cases, cross multiplication is used to solve equations where variables appear on both sides of the equal sign. Here’s an example:

1. Write the equation: x/2 = 3/(x+4).
2. Multiply the top-left (x) by the bottom-right (x+4) and write the product (x(x+4)) on the left side of the equal sign.
3. Multiply the bottom-left (2) by the top-right (3) and write the product (6) on the right side of the equal sign.
4. The equation now looks like this: x(x+4) = 6.
5. Solve for x: x = 2.

Cross Multiply to Compare Fractions

Cross multiplication can be used to compare fractions without having to find a common denominator. By multiplying the numerators and denominators diagonally, you can quickly determine if the fractions are equal, larger, or smaller.

Examples on Cross Multiply

Here are a few examples of cross multiplication:

1. Comparing fractions: 3/4 and 5/6
2. Comparing ratios: 2:3 and 8:12
3. Solving equations with one variable: 5/8 = x/32
4. Solving equations with two variables: a/b = 3/4

Practice Questions on Cross Multiply

Test your understanding of cross multiplication with these practice questions:

1. Compare the fractions 2/3 and 4/6.
2. Compare the ratios 3:4 and 9:12.
3. Solve the equation: 7/8 = x/24.
4. Solve the equation: p/q = 5/10.

Conclusion

In conclusion, cross multiplication is an essential skill for children learning math, as it helps to simplify and compare fractions and ratios effectively. By understanding the concept and mastering the technique, children can approach more complex math problems with increased confidence and proficiency.

At Brighterly, we are dedicated to providing engaging and innovative learning experiences for children, helping them develop a strong foundation in mathematics. We believe that by empowering children with the right tools and techniques, such as cross multiplication, they can unlock their full potential and thrive in their academic journey. So, let’s continue exploring the fascinating world of math together and inspire our young learners to embrace the beauty and power of numbers!

Frequently Asked Questions on Cross Multiply

Can cross multiplication be used for addition or subtraction of fractions?

No, cross multiplication is not suitable for addition or subtraction of fractions. To add or subtract fractions, you need to find a common denominator, which is the lowest common multiple (LCM) of the two denominators. Once you have the common denominator, you can add or subtract the numerators accordingly and keep the denominator the same.

Can cross multiplication be used for mixed numbers?

Yes, cross multiplication can be applied to mixed numbers. However, mixed numbers must first be converted to improper fractions to use this method effectively. To convert a mixed number to an improper fraction, multiply the whole number by the denominator and add the result to the numerator. The resulting value will be the new numerator, and the denominator remains unchanged.

When should I use cross multiplication?

Cross multiplication is a valuable technique for several purposes, including:

1. Comparing fractions or ratios to determine if they are equal, larger, or smaller.
2. Solving equations that involve fractions or ratios with one or two variables.
3. Checking if two fractions are proportional to each other.

Cross multiplication simplifies the process of comparing fractions and solving equations with fractions by eliminating the need to find a common denominator.

Can cross multiplication be used with decimals?

Cross multiplication can be used with decimals, but it’s generally recommended to convert decimals to fractions before applying this technique. This is because working with decimals in cross multiplication can involve more complex arithmetic, and it may be more difficult to compare the results accurately. By converting decimals to fractions, you can maintain the simplicity and effectiveness of cross multiplication. To convert a decimal to a fraction, determine the place value of the decimal, create a fraction with the decimal number as the numerator, and use a denominator of 10, 100, 1000, etc., depending on the decimal’s place value. Then, simplify the fraction if necessary.

Jo-ann Caballes

5 articles

As a seasoned educator with a Bachelor’s in Secondary Education and over three years of experience, I specialize in making mathematics accessible to students of all backgrounds through Brighterly. My expertise extends beyond teaching; I blog about innovative educational strategies and have a keen interest in child psychology and curriculum development. My approach is shaped by a belief in practical, real-life application of math, making learning both impactful and enjoyable.