12.1 Understanding ratio · Unit 12 Ratio and proportion

1 · Teaching slidesuse ← → keys

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2 · Key words

ratioA comparison of two or more amounts, part with part.

simplest formEvery part divided by their highest common factor.

equivalent ratioThe same comparison written with different numbers.

partOne share in a ratio.

proportionA comparison of a part with the whole.

3 · Summary of the sectionrevise from this

A ratio compares amounts part with part: 3 : 2 means 3 of one for every 2 of the other.
Order matters. 3 : 2 is not the same as 2 : 3.
To simplify, divide every part by their highest common factor — exactly like a fraction.
To find an equivalent ratio, multiply or divide every part by the same number.
Before writing a ratio of two measurements, put them in the same units: 40 cm : 1 m becomes 40 : 100 = 2 : 5.
A fraction compares part with whole. In 2 : 3 the first share is 2/5 of the whole, not 2/3.
The total number of parts is the sum of the ratio: 2 : 3 has 5 parts.
That confusion between 2/3 and 2/5 is the commonest error in the whole unit.

PDFPrintable worksheet All 53 questions, space to work PDFWorked answers Full solutions, not just the answer

Video lessons1 videos

Ratio: simplifying

British corbettmaths

Dividing every part by the highest common factor, with the same-units trap included.

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Understanding ratio