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In order to answer this, you can split the calculations into two sections.

Part 1:

Calculate how many times the bacteria divide in 6 hours.

If the bacteria grow for 6 hours, each bacterium will divide 3 times per hour × 6 hours = 18 times.

Part 2:

Calculate the number of bacteria in the population.

Every time the bacteria reproduce, the number doubles. You can use an equation to do this.

Bacteria at the end of the growth period = bacteria at the beginning × 2^{number of divisions} of the growth period

Number of bacteria at the beginning = 1

Number of divisions = 18

Using the above equation: 1 × 2¹⁸ = 1 × 262,144 = 262,144 bacteria

For a higher mark, you could express answers in standard form.

For example, the above answer of 262,144 bacteria can also be written as 2.62 × 10⁵ bacteria.

60 minutes ÷ 1 division every 30 minutes = 2 divisions per hour.

2 divisions × 8 hours = 16 total divisions. Therefore 1 × 2¹⁶ = 1 × 65,536 which is 65,536 bacteria or 6.55 × 10⁴ bacteria in standard form.

60 minutes ÷ 1 division every 20 minutes = 3 divisions per hour.

3 divisions × 2 hours = 6 total divisions, therefore 10 × 2⁶ = 10 × 64 which is

640 bacteria or 6.4 × 10² bacteria in standard form.