MACHINE SHOP CALCULATION - OD1640 - LESSON 2/TASK 2
(3) Rule 2.
To find one unknown mean.
When both extremes and one mean
are known, the unknown mean can be found by dividing the product of the
extremes by the known mean.
EXAMPLE
Find the unknown mean for 15 : 5 = ? : 20.
15 : 5 = X : 20
Let "X" represent the unknown
15 x 20 = 5 x X
mean. Multiply the extremes
300 = 5X
and means. Divide both sides
60 = X
of the equation by 5. The
unknown mean is 60.
(4) Rule 3. To find one unknown extreme. When both means and one extreme
are known, find the unknown extreme by dividing the product of the means by
the known extreme.
EXAMPLE
Find the unknown extreme for ? : 28 :: 2 : 8
Let "X" represent the unknown
extreme. Multiply the means.
Divide by the known extreme.
The unknown extreme is 7.
d.
(1) The ratio 2 : 3 is the inverse of the ratio 3 : 2.
In proportion,
when a ratio is equal to its inverse, the elements are said to be inversely
proportional.
(2) Two numbers are inversely proportional when one increases as the other
decreases.
In this case their product is always the same.
A practical
example of inverse ratio is seen in problems dealing with pulleys.
(a) Rule 1.
The speed of pulleys connected by belts are inversely
proportional to their diameters. The smaller pulley rotates faster then the
larger pulley.
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