Which of the following sentences express correct number style?

a. The professional software package included Web 2.0 applications, but it cost more than $1,200.
b. Mr. Yu predicted that fifty-five point six percent of his customers would return for follow-up services.
c. My salary increased 3 percent last year.

a. The professional software package included Web 2.0 applications, but it cost more than $1,200.

Explanation:

A simple rule for writing a correct number style is that when writing numbers between 1 - 9 ( single digits numbers), you should spell them out. When writing anything larger however or if writing multiple numbers, you should write them as numerals.

This convention can be typically broken every now and then in certain cases. One of those cases is that, if you have a situation where you have 2 numbers in a sentence, you have to use one style for it and if the 2 numbers consist of 1 small number and 1 large number you would preferably have to use numerals.

For this reason Option A which has two numbers written as numerals is correct.

It is worthy of note however, that different styles exist as do Citation Styles and this answer is based on the most rudimentary of said styles.

its kinda hard to answer this when we dont have all the information that we need.