20 identical solid hemispheres of radius 3 cm

Volume of small hemisphere = $ \frac{2}{3}\pi r^3 = \frac{2}{3}\pi (3)^3 = 18\pi $
Volume of 20 hemispheres = $ 20 \cdot 18\pi = 360\pi $
After 20% wastage: $ V_\text{big} = 0.8 \cdot 360\pi = 288\pi $
Volume of big hemisphere =$ \frac{2}{3}\pi R^3 = 288\pi $
$ R^3 = \frac{288 \pi \cdot 3}{2 \pi} = 432 = 216 \cdot 2 = 6^3 \cdot 2 $
$ R = \sqrt[3]{6^3 \cdot 2} = 6 \sqrt[3]{2} $
${6 \sqrt[3]{2}} $