$\frac{11}{20} = \frac{10}{20} + \frac{1}{20} = \frac{1}{2} + \frac{1}{20}$;
$\frac{21}{40} = \frac{20}{40} + \frac{1}{40} = \frac{1}{2} + \frac{1}{40}$;
$\frac{31}{60} = \frac{30}{60} + \frac{1}{60} = \frac{1}{2} + \frac{1}{60}$.
$\frac{1}{20} > \frac{1}{40} > \frac{1}{60}$, значит:
$\frac{11}{20} > \frac{21}{40} > \frac{31}{60}$.
Ответ: $\frac{11}{20}$ − наибольшая дробь.

$\frac{23}{48} = \frac{24 - 1}{48} = \frac{24}{48} - \frac{1}{48} = \frac{1}{2} - \frac{1}{48}$;
$\frac{17}{36} = \frac{18 - 1}{36} = \frac{18}{36} - \frac{1}{36} = \frac{1}{2} - \frac{1}{36}$;
$\frac{35}{72} = \frac{36 - 1}{72} = \frac{36}{72} - \frac{1}{72} = \frac{1}{2} - \frac{1}{72}$.
$\frac{1}{36} > \frac{1}{48} > \frac{1}{72}$, значит:
$\frac{35}{72} > \frac{23}{48} > \frac{17}{36}$.
Ответ: $\frac{35}{72}$ − наибольшая дробь.