Why should '20th of the month' be pronounced as「はつか」in Japanese?

In basic level, you might have learned how to count the dates of the month.

To count from the 11th day of the month, you say「number +にち」。For example, 16th of the month is 「じゅうろく にち」 Easy~.

However, 20th of the month is not 「にじゅうにち」. It’s 「はつか」。Why? It's because 20 is composed of「ふたつ」(2 pieces) and「とお」(10) – in other words, 2 of 10 – and it was pronounced「ふたつとお」originally.

As it was a bit too long, it was shortened to「ふつ」, then later changed to「はつ」, and finally「はつか」. Confusing? I think so too... but this small note is for my students who always ask me 'why~' =)

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