A ratio compares two (or more) quantities. Written
3 : 2, it says "3 of these for every 2 of those".
The order matters: 3 : 2 is not the same as
2 : 3.
A ratio can be part-to-part — comparing one group with another, like
3 : 2. It can also tell you about the whole: add the
parts, 3 + 2 = 5, so there are 5 parts in total and the first quantity
is \tfrac{3}{5} of everything.
A ratio is written a : b — and the order matters.
Add the parts to get the total: a + b parts.
Each part as a fraction of the whole is its share over the total parts:
\dfrac{a}{a + b} and \dfrac{b}{a + b}.
See the parts
Picture the ratio 3 : 2 as a bar of equal blocks — three of one
colour, two of another. Step through to count the parts.
Five equal parts in all: the first colour is \tfrac{3}{5} of the bar,
the second is \tfrac{2}{5}.