구십
gusip · TOPIK 1
ninety
구십 = 구 (nine, 九) + 십 (ten, 十): nine tens = ninety
Sino-Korean numbers stack like multiplication: 구 (gu, nine) times 십 (sip, ten) gives ninety. Same pattern as 이십 (twenty) and 삼십 (thirty). The hanja 九十 matches Chinese jiǔshí and Japanese kyūjū.