(3) *Rule 2*.

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.