2.3 is smaller than 2.6: we can see it just by looking at the digits from left to right. 
now, where does 2[tex] \frac{4}{5} [/tex] stand? 
 its fraction part is equal to
[tex] \frac{4*2}{5*2}=  \frac{8}{10}[/tex] 
do that means that 2[tex]\frac{4}{5} [/tex]=2.8
which is bigger than 2.6
so the final order is 
2.3 2.6 2[tex]\frac{4}{5} [/tex]