2 years. The first time it would go from a quarter of the lake to half. The next year it would double that, taking over the whole lake.
The water lilies will take 2 more years to cover the entire lake after the 10th year when they cover a quarter of its surface. In year 11, they will cover half, and by year 12, they will cover the whole lake. Therefore, the total is 2 more years until full coverage occurs.
;
[tex]5\% \times 1.500.000.000 \\ = \frac{5}{100} \times 1.500.000.000 \\ \\ = \frac{7.500.000.000}{100} = 75.000.000[/tex]