Step-by-step explanation:
What this is saying is...20 is a whole number; but 3/4 is not. What they are saying is, if you have to add a mixed number with a whole number, you can add both of the whole numbers together. Then, you add the 3/4 at the end...because it is not a whole number. You can break apart a mixed number by separating the whole number and the fraction.
Hope this helps :)
