It can be a matter of confusion that how should mathematical numbers be used correctly while writing  an article or any other piece of composition. Following rules offer help in this regard.

• According to most copy editors and English teachers, the numbers zero to nine inclusive should be "written out" – meaning instead of "1" and "2", one would write "one" and "two".

                   Example: "I have two apples." (Preferred)

                   Example: "I have 2 apples."

•         After "nine", one can head straight back into the 10, 11, 12, etc., although some write out the numbers until "twelve".

                   Example: "I have 28 grapes." (Preferred)

                   Example: "I have twenty-eight grapes."

•         Another common usage is to write out any number that can be expressed as one or two words, and use figures otherwise.

                   Examples:

                   "There are six million dogs." (Preferred)

                   "There are 6,000,000 dogs." (The reader might end up counting                          zeroes!!!!)

                   "That is one hundred (and) twenty-five oranges."

                   "That is 125 oranges." (Preferred)

•         Numbers at the beginning of a sentence should also be written out.

          "Two of my friends are waiting for me." (Preferred)

          "2 of my friends are waiting for me."

The above rules are not always used.

•         In literature, larger numbers might be spelled out.
•         On the other hand, digits might be more commonly used in technical or financial articles, where many figures are discussed.
•         In particular, the two different forms should not be used for figures that serve the same purpose.

                   Example, it is inelegant to write, "Between day twelve and day                           15 of the study, the population doubled."