20. \(\frac{15}{4}\cdot\frac{6}{5}\)

Сократим \(15\) и \(5\), затем \(6\) и \(4\): \(\frac{15}{4}\cdot\frac{6}{5}=\frac{9}{2}=4\frac{1}{2}\).

Ответ: \(4\frac{1}{2}\).