$a\text{)}\sqrt{4}\cdot \sqrt{25}=2\cdot 5=10$  

$b\text{)}\sqrt{3\frac{15}{16}}:\sqrt{1\frac{3}{4}}=\sqrt{\frac{63}{16}}:\sqrt{\frac{7}{4}}=\sqrt{\frac{{\sout{63}}^9}{{\sout{16}}_1}\cdot \frac{{\sout{4}}^1}{{\sout{7}}_1}}=\sqrt{9}=3$ $b\text{)}\sqrt{3\frac{15}{16}}:\sqrt{1\frac{3}{4}}=\sqrt{\frac{63}{16}}:\sqrt{\frac{7}{4}}=\sqrt{\frac{{\sout{63}}^9}{{\sout{16}}_1}\cdot \frac{{\sout{4}}^1}{{\sout{7}}_1}}=\sqrt{9}=3$  

$c\text{)}\sqrt{1,21}\cdot \sqrt{100}=1,1\cdot 10=11$  

$d\text{)}\sqrt[3]{0,25}:\sqrt[3]{16}=\sqrt[3]{0,25:16}=\sqrt[3]{\frac{1}{4}\cdot \frac{1}{16}}=\sqrt[3]{\frac{1}{64}}=\frac{1}{4}$  

$e\text{)}\sqrt{0,2}\cdot \sqrt{0,1}\cdot \sqrt{200}=\sqrt{0,2\cdot 0,1\cdot 200}=4$  

$f\text{)}\sqrt[3]{2}:\sqrt[3]{54}=\sqrt[3]{2:54}=\sqrt[3]{2\cdot \frac{1}{54}}=\sqrt[3]{\frac{1}{27}}=\frac{1}{3}$