11) 2 1/4 : 1 4/11 − 3/8 : 7/8;

Преобразуем смешанные числа: \(2\frac{1}{4}=\frac{9}{4}\), \(1\frac{4}{11}=\frac{15}{11}\).

\(\frac{9}{4}:\frac{15}{11}=\frac{9}{4}\cdot\frac{11}{15}=\frac{33}{20}\).

\(\frac{3}{8}:\frac{7}{8}=\frac{3}{7}\).

\(\frac{33}{20}-\frac{3}{7}=\frac{231}{140}-\frac{60}{140}=\frac{171}{140}=1\frac{31}{140}\).

Ответ: \(1\frac{31}{140}\).