1.
eight hundred (800)
The number 800. 八 + 百 with a double sound change: はち becomes はっ and ひゃく becomes ぴゃく.
八 + 百, pronounced はっぴゃく — never はちひゃく.
Pronunciation
- 三百: ひゃく → びゃく
- 六百: ろく → ろっ, ひゃく → ぴゃく
- 八百: はち → はっ, ひゃく → ぴゃく
The others are regular.
When the same two characters mean 'a great many' rather than the exact number, they are read 八百: 八百屋 (greengrocer), 八百万の神 (the countless gods), 八百長 (a fixed match). Read はっぴゃく only when the actual figure 800 is meant.