Let's say your teacher asked you to rewrite 21/5 as a mixed number.
The first thing you would do is divided the numerator by the denominator.
21/5 = 4 with a remainder of 1. (4 * 5 = 20 with one left over to get to 21.)
Step 2; The whole number is your whole number in the mixed number and the remainder becomes the numerator of the fraction. Keep in mind that the denominator ALWAYS stays the same!
