Year 9: Finding values based on ratios

This cheat sheet helps you find values when working with ratios.

What is a Ratio?

A ratio compares two quantities. It can be expressed in several ways:

• Part-to-Part: Describes the relationship between specific parts of something. Example: 2:3 (2 parts to 3 parts)
• Part-to-Whole: Describes the proportion of a part compared to the whole. Example: 2:5 (2 parts out of a total of 5 parts)

Finding Missing Values Using Ratios

1. Given a Ratio and a Total:

If you know a ratio and the total value of the parts, you can calculate a single value.

Example: A recipe calls for a ratio of flour to sugar of 3:2. The total amount of ingredients is 140g. How much flour is needed?

Steps:

1. Total Ratio Parts: Add the numbers in the ratio: 3 + 2 = 5
2. Fraction of Flour: The ratio is 3:5, so the flour represents 3/5 of the total.
3. Calculate Flour: (3/5) * 140 g = 84 g

2. Given a Ratio and a Known Value:

If you know one part and the ratio, you can calculate the other part.

Important Note:

Always simplify your ratios. Expressing ratios in their simplest form helps avoid errors.