Within $x$ weeks, water pollution has increased from 2 percent to 75 percent. It holds that

$$
2 \cdot 2^{\frac{x}{3}}=75, \quad x=3 \frac{\lg 75-\lg 2}{\lg 2} \approx 16
$$

Within 16 weeks, water pollution has increased to 75 percent.
## 224. Math Puzzle $1 / 84$

Due to water pollution, algae have settled in a pond, doubling in size within three weeks. Initially, only 2 percent of the pond was contaminated. When did these 2 percent grow to 75 percent?