a. $7\frac{9}{10} = \frac{79}{10}$;    $4\frac{9}{10} = \frac{49}{10}$

=> $7\frac{9}{10}$ > $4\frac{9}{10}$;   

b. $6\frac{3}{10}=\frac{63}{10}= \frac{567}{90}$,      $6\frac{5}{9}=\frac{59}{9}=\frac{590}{90 }$

=>$6\frac{3}{10}$ < $6\frac{5}{9}$