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.

Two variables are inversely proportional (or varying inversely, or in inverse variation, or in inverse proportion or reciprocal proportion) if one of the variables is directly proportional with the multiplicative inverse (reciprocal) of the other, or equivalently if their product is a constant. It follows that the variable y is inversely proportional to the variable x if there exists a non-zero constant k such that

The constant can be found by multiplying the original x variable and the original y variable.

Basically, the concept of inverse proportion means that as the absolute value or magnitude of one variable gets bigger, the absolute value or magnitude of another gets smaller, such that their product (the constant of proportionality) is always the same. (Our solved example in mathguru.com uses this concept)

For example, the time taken for a journey is inversely proportional to the speed of travel; the time needed to dig a hole is (approximately) inversely proportional to the number of people digging.

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

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