Ratios & Proportions
Set two equal ratios and cross-multiply.
The rules
- A ratio compares two quantities. Keep the same order on both sides.
- In a proportion a/b = c/d, cross-multiply: ad = bc.
- For part-to-part ratios, add the parts to get the whole.
- Use units to check your setup.
Watch out: Mixing the order (miles/hours on one side and hours/miles on the other).
Worked example
A recipe uses 3 cups of flour for 24 cookies. How many cups for 40 cookies?
- 3/24 = x/40.
- 24x = 120, so x = 5.
Answer: 5
Try it yourself
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