Ratio and Proportion

Machine Shop Calculation
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General. Ratio and proportion are methods for reducing the confusion

and minimizing the possibilities of error in working arithmetic problems. A

working knowledge of these methods makes it easier to solve many shop

problems. The language of ratio and proportion is mostly a sign language.

Letters and symbols are commonly used in place of long numbers and represent

unknown quantities and values.

Ratio.

(1) Ratio is the relation which one quantity bears to another quantity of

the same kind.

It is used extensively in shop work.

Shop drawings or

blueprints are generally drawn to scale. Scale means one figure is used to

represent another. Usually a small figure represents a larger figure. For

example, on a blueprint 1 inch might represent 1 foot.

(2) The two numbers used in the ratio are called the "terms" of the ratio.

The first number of a ratio is called the antecedent; the second number is

called the consequent. The consequent is the divisor. The colon (:) is the

sign of ratio and means "is to." Thus, 3 : 5 reads "3 is to 5." It is in

effect a dividing sign without the dividing (-) line.

Such other

expressions as "in the same ratio", "in the same proportion', or "pro rata"

all have the same meaning.

(3) The ratio of one number to another is really the quotient of the first

number divided by the second number.

EXAMPLE

Determine the ratio of the expression 8 : 2.

2=4

Divide 8 by 2.

Thus, the ratio or value of 8 to 2

is 4.

(4) The value of a ratio is not changed by either multiplying or dividing

both terms by the same number.