Proportions for Kids: Equal Ratios, Cross Products and Missing Numbers (Grade 7 Math)

Hi, I'm Luna from DoodleBeez. Today we are solving proportions. A proportion says that two ratios are equal. For example, one over two equals three over six, because both show the same relationship.

To check if two ratios form a proportion, find the cross products. For two over three and eight over twelve, two times twelve is twenty four, and three times eight is twenty four. They match, so it is a proportion.

Now try three over four and five over six. Three times six is eighteen, but four times five is twenty. The cross products are different, so these two ratios are not equal.

We can find a missing number. Three over five equals x over twenty. Five times four is twenty, so multiply the top by four as well. Three times four is twelve, so x equals twelve.

Or you can cross multiply. Four over six equals ten over x. Four times x equals six times ten, which is sixty. Divide sixty by four, and x equals fifteen.

Proportions solve real problems. If three tickets cost twenty seven dollars, how much do five tickets cost? Each ticket costs nine dollars, so five tickets cost five times nine, which is forty five dollars.

Now it's your turn. Two over seven equals eight over x. What is x? Pause and try. Two times four is eight, so multiply seven by four as well. Seven times four is twenty eight, so x equals twenty eight.

Great job! Remember, a proportion is two equal ratios. Check with cross products, and find a missing number by scaling or cross multiplying. Practice proportions in Middle School Math at doodlebeez.com. See you next time!