This is a method
of carrying out a calculation to find the value of a number of items
by first finding the cost of one of them.

For example, suppose that 3 tennis balls cost $5.52 and
we need to find the cost of 5 tennis balls. We can find first
find the cost of 1 ball as $5.52 / 3 = $1.84, then of the
5 balls by $1.84 x 5 = £9.20.

The same approach can be used to find how far a train will travel
in 25 minutes if in 8 minutes it covers 11.2 miles, assuming it
travels at the same speed. In 1 minute the train travels 11.2 ÷
8 = 1.4 miles, so in 25 minutes it will travel 1.4 x 25 = 35 miles.
For instance if we know that a and b are proportional to one another,
and that when a = 7, b = 10, and we are required to find the value of
a when b = 7, then we may proceed as follows: a = 7 when b = 10, so a = 7/10
when b = 1, so a = 49/10 when b = 7. This is called a unitary method.