20. 21/√3 ⋅ ³√(1 1/3)

20. Преобразуем выражение.

$$\frac{21}{\sqrt{3}} \cdot \sqrt[3]{1 \frac{1}{3}} = \frac{21}{\sqrt{3}} \cdot \sqrt[3]{\frac{4}{3}} = \frac{21}{\sqrt{3}} \cdot \frac{\sqrt[3]{4}}{\sqrt[3]{3}} = \frac{21}{\sqrt{3}} \cdot \frac{\sqrt[3]{4}}{\sqrt[3]{3}} \cdot \frac{\sqrt{3}}{\sqrt{3}} \cdot \frac{\sqrt[3]{3^2}}{\sqrt[3]{3^2}} = \frac{21 \cdot \sqrt{3} \cdot \sqrt[3]{4 \cdot 3^2}}{3 \cdot 3} = \frac{21 \cdot \sqrt{3} \cdot \sqrt[3]{36}}{9} = \frac{7 \cdot \sqrt{3} \cdot \sqrt[3]{36}}{3}$$.

Ответ: $$\frac{7 \cdot \sqrt{3} \cdot \sqrt[3]{36}}{3}$$