Lesson 7 of 15
Two ratios that are equal
Show on a big screenTwo cement mixes are labelled 3 : 4 and 9 : 12. Do they match?
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Step 1 of 3
- Two ratios: 3 : 4 and 9 : 12.
- Outer numbers: 3 × 12 = 36.
- Inner numbers: 4 × 9 = 36 — a match.
The rule
Yes, they match: multiplying across gives 36 both times.
Two equal ratios are in proportion. Write them as a : b :: c : d. Read it as a is to b as c is to d. Multiply across: a × d and b × c. Here a and d are the outer numbers. The inner numbers are b and c. Equal products mean a match.
- 3 : 4 and 8 : 10: 3 × 10 = 30 and 4 × 8 = 32. These differ, so no match.
- To find ? in 3 : 4 = 15 : ?, multiply across. 3 × ? = 4 × 15 = 60, so ? = 60 ÷ 3 = 20.
Try the lab to finish this lesson.