Remember that fractions are used to count a part of something.
|of this square is red.|
Mixed numbers are used when you need to count whole things AND parts of things at the same time.
Check this out:
How much of these squares is red?
There are 3 whole squares and of another square...
We write it like this:
and read it like "three and one fourth."
But, we don't put the "+" in... This is why we say the "and."