Explanation:

- We need to find how the fifth number, 54, relates to other numbers.

- In the series: 31:3, the relationship is found through division, where 31 ÷ 10 = 3.1, rounded down gives us 3.

- In the series: 75:35, the relationship is also based on division, where 75 ÷ 2 = 37.5, rounded down gives us 35.

- We now apply this logic: 54 ÷ X = ?, where X should match one of the options.

- Testing: 54 ÷ 3 = 18 and 54 ÷ 2.7 = 20 (approximately).

Option 4: 20 is correct.

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