Remember that fractions are used to count a part of something.
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 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?
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There are 3 whole squares and 
of another square...
We write it like this:
and read it like "three and one fourth."
It's really
But, we don't put the "+" in... This is why we say the "and."