The surface of the Earth consists of $70\%$ water and $30\%$ land. Two fifths of the land are deserts or covered by ice and one third of the land is pasture, forest, or mountain; the rest of the land is cultivated. What is the percentage of the total surface of the Earth that is cultivated?
The fraction of land that is cultivated is

$$
1-\frac{2}{5}-\frac{1}{3}=\frac{15-6-5}{15}=\frac{4}{15}
$$

Since land occupies 3/10 of the total surface area of the Earth, it follows that the cultivated area is $\frac{4}{15} \times \frac{3}{10}=\frac{2}{25}$, that is, $\frac{2}{25}=\frac{2}{25} \times \frac{4}{4}=\frac{8}{100}=8\%$ of the Earth's surface.