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Japanese pronounces a language in a basis "kana-50-on".
But there is a word to change according to an idiomatic rule.
It is a numeral.
Japanese numeral is a word showing the meaning by number and unit character.
The unit character varies with object word.
And a pronunciation varies with any number, too.
This site has list of the change rule table, and introduces it.
When romanization and pronunciation varies, I show pronunciation.