사십
sasip · TOPIK 1
forty
사십 = 사 (four, 四) + 십 (ten, 十): four tens = forty
Sino-Korean numbers stack like multiplication: 사 (four) placed before 십 (ten) means four tens. The same pattern gives 이십 (20), 삼십 (30) and 오십 (50), so knowing 사 and 십 gives you 사십 for free.