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    Ratio, Proportion & Rates of Change

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    Lesson step 1 of 4Ratio

    Ratio

    A ratio compares quantities. To simplify a ratio, divide all parts by their HCF — just like simplifying a fraction.

    Sharing in a ratio is a very common exam question. Here's the method:
    1. Add up the total parts
    2. Divide the total amount by the total parts to find the value of one part
    3. Multiply each ratio part by the value of one part

    You might also be given one share and asked to find the total. In that case, work out the value of one part first, then multiply by the total number of parts.

    To combine two ratios that share a common element (e.g., a:b and b:c), find a common multiple for that element and scale both ratios so that the common element has the same value in each ratio. Then, you can write the combined ratio.

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    Key Terms

    Direct proportion
    When two things increase together at the same rate — double one and you double the other.
    Inverse proportion
    When one thing goes up, the other goes down — double one and you halve the other.
    Compound measure
    A measurement that combines two different units — like speed (miles per hour) or density (grams per cm³).