Environmental Engineering Reference
In-Depth Information
9
Ratios and Proportions
A
B
C
D
=
or
AB
:
=
CD
:
R ATIOS
A ratio is the established relationship between two numbers. For example, if some-
one says, “I'll give you four to one the Redskins over the Cowboys in the Super
Bowl,” what does that person mean? Four to one, or 4:1, is a ratio. If someone
gives you 4 to 1, it's that person's $4 against your $1. As another, more pertinent
example, if an average of 3 ft
3
of screenings are removed from each million gallons
of wastewater treated, the ratio of screenings removed (ft
3
) to treated wastewater
(MG) is 3:1. Ratios are normally written using a colon (such as 2:1) or as a fraction
(such as 2/1).
PROPORTIONS
A proportion
is a statement that two ratios are equal. For example, 1 is to 2 as 3 is to
6, so 1:2 = 3:6. In this case, 1 has the same relationship to 2 that 3 has to 6; in other
words, 1 is half the size of 2, and 3 is half the size of 6. Or, alternatively, 2 is twice
the size of 1, and 6 is twice the size of 3.
WORKING WITH RATIOS AND PROPORTIONS
When working with ratios and proportions, the following key points are important
to remember:
1. One place where fractions are used in calculations is when ratios and pro-
portions are used, such as calculating solutions.
2. A ratio is usually stated in the form “A is to B as C is to D,” and we can
write it as two fractions that are equal to each other:
A
B
C
D
=
129
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