forty; 40
The number forty. Reading よんじゅう is the modern standard; しじゅう is a less common Sino-Japanese reading still used in some fixed contexts like age or traditional counting. よそ is obsolete.
彼は今年で四十歳になります。
He will be forty years old this year.
このクラスには四十人の学生がいます。
There are forty students in this class.
三十 means thirty while 四十 means forty; both follow the same Ｘ十 pattern for multiples of ten and can be confused when read fast.
Standard kanji spelling, frequent in vertical text and formal writing.
Formal daiji (大字) spelling used in financial instruments to prevent forgery.
Arabic numeral spelling, very common in horizontal writing and practical contexts.
Uncommon kanji-numeral hybrid spelling; mostly seen in legal or financial documents.
Archaic single-character form for forty; not used in modern Japanese.
Compound of 四 (four) and 十 (ten). The reading よんじゅう uses the native Japanese よん for four and the Sino-Japanese じゅう for ten; しじゅう uses the Sino-Japanese reading し for four.