7th Grade Math · Proportions
Unknown denominator Worksheets
Fresh practice, ready to print. Answer key included.
Proportions such as 5/x = 15/21.
Make it yours
Or start with a ready-made version.
How it works: a quick refresher
The rules this worksheet practices, then one problem worked from start to finish.
| Strategy | How it works |
|---|---|
| Cross-multiply | For x/4 = 9/12, multiply diagonally: 12 × x = 4 × 9. Then 12x = 36, so x = 3. |
| Or simplify one fraction first | 9/12 divides by 3 to give 3/4. Then x/4 = 3/4, so x = 3. Use this when a common factor is easy to see. |
| A missing denominator flips the question | 5/x = 15/21. Cross-multiply: 15 × x = 5 × 21, so 15x = 105 and x = 7. |
| The letter can sit on either side | x/4 = 9/12 and 9/12 = x/4 ask for the same number. |
| Check by substituting | Put the number back in. Both fractions should be the same ratio. 3/4 = 9/12. |
Worked example
Solve x/4 = 9/12.
- 1Cross-multiply: 12 × x = 4 × 9.
- 212x = 36.
- 3Divide by 12: x = 3.
- 4Check: 3/4 = 9/12.
Ready-made versions
Three fixed versions you can link to or share. Each is a PDF with its own answer key.
- Unknown denominator Worksheets, version 1Worksheet PDFAnswer key PDF
- Unknown denominator Worksheets, version 2Worksheet PDFAnswer key PDF
- Unknown denominator Worksheets, version 3Worksheet PDFAnswer key PDF
Grade level and standard
Written for 7th grade math (ratios) and aligned to CCSS 7.RP.A.2. It also works as review in later grades.
Common questions
- Is there an answer key?
- Yes. Every worksheet has a matching answer key with each answer worked on the same grid or layout as the question. The worksheet ID printed at the bottom of each page pairs a sheet with its key.
- Why is the answer never 1?
- An answer of 1 usually means both fractions were already equal to 1, which is a glance rather than a proportion to solve. These problems take a real cross-multiply.
- Can my child write the slash as a fraction bar?
- Yes. x/4 means the same as a fraction with x on top and 4 on the bottom, and many students find the fraction bar easier to work with.
- How many different versions are there?
- About a million per skill. Each time you press the button the problems are generated fresh, so you can print a different version for every student, a retake, or another night of practice.
- Are the worksheets free?
- Yes. They are free to print and copy for your home or classroom, with no sign-up.