Why do you multiply the whole number by the denominator to get a mixed fraction?

You don't. But you do. When converting a mixed number into an improper fraction. A mixed fraction contains a whole number and a proper fraction, for example 23⁄4. To convert this to an 'improper' fraction, the whole number is multiplied by the denominator and added to the numerator to obtain the new numerator. See. but WHY? The denominator remains unchanged (unless, of course, you wish to simplify the fraction). This is unrelated! At no point did you actually provide an answer to this question.

Continuing with the original example, for 23⁄4, (2 · 4 + 3) = 11, so the new fraction is 11⁄4.

Ps. Anything in italics has been improved by another person.