15. (1 11/16 - 3 3/8) * 4

$$ (1\frac{11}{16} - 3 \frac{3}{8}) \cdot 4 = (\frac{16 + 11}{16} - \frac{24 + 3}{8}) \cdot 4 = (\frac{27}{16} - \frac{27}{8}) \cdot 4 = (\frac{27}{16} - \frac{27 \cdot 2}{16}) \cdot 4 = (\frac{27 - 54}{16}) \cdot 4 = \frac{-27}{16} \cdot 4 = \frac{-27 \cdot 4}{16} = \frac{-27}{4} = -6 \frac{3}{4}$$

Ответ: $$-6 \frac{3}{4}$$