- \(0.5\cdot40-\frac13\cdot6=20-2=18\).
- \(\sqrt{0.25\cdot81}=\sqrt{20.25}=4.5\).
- \(\sqrt{6^2\cdot2^8}=6\cdot2^4=96\).
- \(\sqrt{20}\cdot\sqrt5-\frac{\sqrt{68}}{\sqrt7}=\sqrt{100}-\sqrt{\frac{68}{7}}=10-\frac{2\sqrt{119}}7\).
Ответ: 1) \(18\); 2) \(4{,}5\); 3) \(96\); 4) \(10-\frac{2\sqrt{119}}7\).