18) \((\frac{11}{12} + \frac{11}{20}) \cdot \frac{15}{8} = (\frac{55}{60} + \frac{33}{60}) \cdot \frac{15}{8} = \frac{88}{60} \cdot \frac{15}{8} = \frac{22}{15} \cdot \frac{15}{8} = \frac{11}{1} \cdot \frac{1}{4} = \frac{11}{2} = 2\frac{3}{4}\)
Ответ: 11/2