Answer:
54 million
Explanation:
You want the population after 90 minutes if it starts at 2 million and triples every 30 minutes.
Triples
After 30 minutes, the population is ...
3 × 2 million = 6 million
After another 30 minutes (60 minutes, total), the population is ...
3 × 6 million = 18 million
After a total of 90 minutes, the population has tripled again to ...
3 × 18 million = 54 million
The number after 90 minutes is about 54 million.