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

Bacteria divide every 20 minutes, and will divide three times every hour (60 ÷ 20 = 3).

3 × 6 = 18

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

2 divisions per hour. 2 × 8 = 16 total divisions, therefore 1 × 2¹⁶ = 1 × 65,536 which is 65,536 bacteria or 6.55 × 10⁴ bacteria (standard form).

3 divisions per hour. 3 × 2 = 6 total divisions, therefore 10 × 2⁶ = 10 × 64 which is 640 bacteria or 6.4 × 10² bacteria (standard form).