Решение:
\[ \frac{3}{8} - \frac{4}{15} = \frac{3 \cdot 15}{8 \cdot 15} - \frac{4 \cdot 8}{15 \cdot 8} = \frac{45}{120} - \frac{32}{120} = \frac{45 - 32}{120} = \frac{13}{120} \]
\[ 2\frac{4}{13} \cdot \frac{13}{120} = \frac{2 \cdot 13 + 4}{13} \cdot \frac{13}{120} = \frac{26 + 4}{13} \cdot \frac{13}{120} = \frac{30}{13} \cdot \frac{13}{120} = \frac{30}{120} = \frac{1}{4} \]
\[ 11 : 5\frac{1}{2} = 11 : \frac{5 \cdot 2 + 1}{2} = 11 : \frac{11}{2} = 11 \cdot \frac{2}{11} = 2 \]
\[ \frac{1}{4} - 2 = \frac{1}{4} - \frac{8}{4} = \frac{1 - 8}{4} = -\frac{7}{4} \]
Ответ:
\[ -\frac{7}{4} \]