Explanation:

In mathematics, two variable quantities are proportional if one of them is always the product of the other by a constant quantity, called the coefficient of proportionality, or the proportionality constant. In other words, x and y are proportional if the ratio  is constant. We also say that one of the quantities is proportional to the other. For example, if the speed of an object is constant, it travels a distance proportional to the travel time.

The mathematical symbol '∝' is used to indicate that two values are proportional. For example, A ∝ B.

Given two variables x and y, y is (directly) proportional to x (x and y vary directly, or x and y are in direct variation) if there is a non-zero constant k such that

The relation is often denoted

and the constant ratio

is called the proportionality constant or constant of proportionality.

(Our solved example in mathguru.com uses this concept)

Examples

1.  If an object travels at a constant speed, then the distance traveled is proportional to the time spent traveling, with the speed being the constant of proportionality.

2.  The circumference of a circle is proportional to its diameter, with the constant of proportionality equal to π.

3.  On a map drawn to scale, the distance between any two points on the map is proportional to the distance between the two locations that the points represent, with the constant of proportionality being the scale of the map.

4.  The force acting on a certain object due to gravity is proportional to the object's mass; the constant of proportionality between the mass and the force is known as gravitational acceleration.

http://en.wikipedia.org/wiki/Proportionality_(mathematics)

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