Solving a Proportion

Part of the Decimals, Percents & Ratios section of Coddy's Pre-Algebra journey. Lesson 47 of 56.

A proportion is a statement that two ratios are equal, like 3/5 = 9/15. When one of the four numbers is missing, the proportion tells you what it must be. Take n/4 = 21/28.

Look for the pair of matching numbers you know completely: here the two bottoms, 4 and 28. 28 is 4 times 7, so the right ratio is the left one scaled up by 7. Going from the right side back to the left divides by 7, so divide the top as well: 21 divided by 7 is 3, and n = 3.

The scale factor can come from the bottoms or the tops, whichever pair is complete, and it may be a multiplication or a division depending on which side the missing number sits. Check your claim by simplifying: 3/4 and 21/28 are the same ratio.

Challenge

Find the missing number n in the proportion on the board. Find the scale factor between the two known bottoms, use it on the top, then type your claim, like n = ...

Try it yourself

n/6 = 15/18

Solve for n