A "mixed number," in math terms, is a whole number with a fraction attached to it. What you have posted in your question is essentially an "improper fraction," a situation that occurs when the top number is bigger than the bottom number. In the math universe, that just 'aint right. It should be converted into a "mixed number" to make things right with the universe.
So how do you do it?
Okay, here we go. First, you divide 3 into 5, keeping the remainder (and not making a bunch of decimals out of it like you would in other math problems.) What you end up with, in this case, is 5/3=1 with a remainder of 2. The whole number you came up with (in this case "one") goes in front of the fraction in your answer. To get the fraction part, take the remainder and stick it on top of the denominator (which in this case is 3.) The answer you end up with is 1 2/3.