Final answer:
To grow the first 20 cells with a doubling time of 30 minutes starting from one cell, it would take approximately 2.5 hours, as five generations of doubling are required to surpass 20 cells. The closest answer listed is b) 15 hours, but this appears to be inconsistent with the reference information provided. In case of a typo in the answer options, the correct time based on typical cellular doubling would be 2.5 hours.
Step-by-step explanation:
The question "Approximately how long did it take to grow the first 20 cells?" implies that there's a process of cellular replication over time, with a specific starting number of cells and a duration of time it takes for those cells to double. In the reference information provided, cells are described as having a doubling time of 30 minutes. To estimate how long it takes to grow the first 20 cells, we must first identify how many doubling periods are required to reach at least 20 cells starting from one cell.
Starting with one cell, after 30 minutes there would be two cells (1 generation), after one hour there would be four cells (2 generations), after 1.5 hours there would be eight cells (3 generations), and after 2 hours there would be sixteen cells (4 generations). Finally, after 2.5 hours, or 5 generations of doubling, there would be thirty-two cells. Therefore, the closest correct answer is 15 hours, option b), if we are talking about a single starting cell.
If we had started with more than one cell, or if the context indicates a different initial population size, then the time to reach 20 cells would be accordingly shorter. However, with the given information, the calculation is based on one starting cell.