Read a list of rules regarding the use of numbers in formal writing available here: http://www.grammarbook.com/numbers/numbers.asp

There's a practice quiz available at the bottom of the page, if you're interested in trying it out.

Your assignment:

Identify 5 sentences from your own essay where you've used a number. (Preferably, these will not be examples only of in-text citation with page numbers, but contain numbers in the content of the sentences. Also, do not choose sentences that are quoted from an outside source, since you can't change the way those numbers are written.) These do not have to be consecutive sentences in your essay. Copy those 5 sentences into a new document.
Using the guidance of the website, include an explanation as to what rule governs the way the number is written in each sentence.
If you feel that an original use of a number was incorrect in your essay submission, correct the sentence now.

Sample:

1) Original sentence: Since my father’s passing in the 1990’s, his entire side of my family will most likely remain a mystery.

The website says "You may also express decades in complete numerals. Again, don't use an apostrophe between the year and the s."

Revision: Since my father’s passing in the 1990s, his entire side of my family will most likely remain a mystery.

2) Original: On adopting a child of another race, one author states that “Of course your own race hasn’t changed at all, but the race of one family member affects all family members."

The website says to "Spell out single-digit whole numbers. Use numerals for numbers greater than nine." So this version is correct.

3) Original: More Caucasian couples are adopting children of other races and cultures than there were 30 years ago.

The website says to "Spell out single-digit whole numbers. Use numerals for numbers greater than nine." So this sentence is correct.

**Note: this example only offers 3 sentences, while your own submission will need 5.**