Primary mathematics/Proportions

Ratios are similar to fractions. $$x:y= \frac{x} {x+y}$$. Remember, The proportion x:y does not equal the fraction $$\frac{x} {y}$$.

For example, if the ratio of boys to girls in a classroom is 1:2, that does not mean half of the class is boys and the other half is girls. It means that $$\frac{1} {3}$$ of the class is boys and $$\frac{2} {3}$$ of the class is girls. (But that means that the number of boys is half the number of girls.)

But remember that the fraction only applies to the sum of the two sides $$\left(x+y\right)$$, and can be wrong if there are more than 2 things.

For example, if there is a fruit basket that has 16 fruits, and the ratio of apples to oranges is 3:1, that doesn't always mean that there are 12 apples $$\left(\frac{3} {4} \times 16\right)$$ and 4 oranges $$\left(\frac{1} {4} \times 16\right)$$. There could be 12 bananas in the fruit basket, which means that there are only 3 apples and 1 orange.