There are four examples given of fractions in the end of the lesson : 

 tres cuartos, 

dos decimos,

 tres octavos, 

 un septimo  

These examples are supposed to show the need for plurals when the numerator (top number) is more than one.  Unfortunately all the numbers choosen ( dos and tres) end in "s".  I found myself somewhat confused, and wondering if this meant that the top number should also  have an "s", por ejemplo sietes ochos instead of siete ochos.   

 It would be clearer if you used included a fraction that did not end in "s";  por ejemplo  cuarto octavos instead of tres octavos.