Answer 1

Let x be the number of days taken by the earth to pick up

\[4 . 8 \times {10}^8\]  kg of dust.

 

\[\text{ Since the amount of dust picked up by the earth and the number of days are in direct variation, we get: } \]
\[\frac{15}{x} = \frac{1 . 2 \times {10}^8}{4 . 8 \times {10}^8}\]
\[ \Rightarrow x = 15 \times \frac{4 . 8}{1 . 2}\]
\[ \Rightarrow x = 60\]

Thus, the required number of days will be 60.