Ratio and Proportion — Formula Sheet

a : b = \frac{a}{b}

a : b = (a \times k) : (b \times k)

a : b :: c : d \Rightarrow a \times d = b \times c

a : b = b : c \Rightarrow b^2 = a \times c

Value of 1 unit = \frac{\text{Total Value}}{\text{Number of Units}}

Value of n units = (Value of 1 unit) \times n

In a proportion a:b = c:d, the product of means equals the product of extremes (b×c = a×d).

Ratio of boys to girls = Number of boys / Number of girls.

For example, 2:3 = 4:6, so they are in proportion.