а) \(\sqrt{0,64}-\sqrt{0,04}\); б) \(3\cdot\sqrt{0,16}\); в) \(\sqrt{900}\cdot\sqrt{100}\); г) \(\sqrt{5^{-7}\cdot7^{-2}}\).

1. \(\sqrt{0,64}-\sqrt{0,04}=0,8-0,2=0,6\).
2. \(3\cdot\sqrt{0,16}=3\cdot0,4=1,2\).
3. \(\sqrt{900}\cdot\sqrt{100}=30\cdot10=300\).
4. \(\sqrt{5^{-7}\cdot7^{-2}}=\sqrt{\frac{1}{5^7\cdot7^2}}=\frac{1}{5^{7/2}\cdot7}=\frac{1}{875\sqrt{5}}=\frac{\sqrt{5}}{4375}\).

Ответ: а) \(0,6\); б) \(1,2\); в) \(300\); г) \(\frac{\sqrt{5}}{4375}\).