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7 1/2, because year zero belongs to previous decade.

Years are 1 indexed, there never was year 0. It was year 1 BC and then straight to year 1 AD. If it's 1 indexed, than the last year of the decade is 10, otherwise the first decade would be only 9 years long.

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There is no one way of doing this. What you write is correct, but most people use the (IMO simpler) 3rd digit rule: the decade is decided by the third digit of the year. So 2020, 2021, ..., 2028, 2029 belong to the same decade, as do 2030, 2031, ..., 2038, 2039.

This indeed causes problems for the 1 - 9 AD decade, which only has 9 years this way, but most people won't care because we live in 2023.

>but most people use the (IMO simpler) 3rd digit rule: the decade is decided by the third digit of the year

Most people using something doesn't mean it's right. Most people could say PI is 3, because it's easier to remember, but it don't change a thing.

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