New answers tagged numbers
The counting system for large numbers is almost the same as Western numbers, except that digits are delimited into blocks of four instead of three. You add "thousand, million, billion, trillion, ..." for each 3 digit, and in Japanese, we add "万, 億, 兆, 京【けい】, ..." for each 4 digit. The "correct" format to write long numbers depends on the situation. ...
As spoken I think you're right on the money with 三兆四千七百六十五億二千百八十九万三千四百二十一 I would also give the possible option of writing it with an extra 「一」for visual clarity and to look more classical. 三兆四千七百六十五億二千（一）百八十九万三千四百二十一
I think it's just a number. In theory, you could use it any time you wanted to say 1040, like if you were describing the number of ways you could choose 45 objects out of a bag of 160 objects, but in practice I doubt it would be written or spoken that way. Japanese has lots of words for big numbers. Actually, English has some too! You could equally ask ...
It is legit. From 大辞林 ⑦ 数の単位。澗（かん）の1万倍，すなわち10の40乗。 〔塵劫記〕 Also, from the Wikipedia JP page on "正 (数)". 正（せい）は漢字文化圏における数の単位の一つ。正がいくつを示すかは時代や地域により異なるが、現在では1040を示す。 You can see a list of these numerical terms for large numbers on the Wikipedia JP page on "命数法". Oh, here's an English version. That said, while 大辞林 has the corresponding entry for each ...
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