오십
osip · TOPIK 1
fifty (Sino-Korean numeral)
오십 = 오 (five) + 십 (ten): five tens make fifty
Sino-Korean numbers stack like multiplication: 오 (五, five) in front of 십 (十, ten) means five times ten. The same pattern gives 이십 (twenty) and 삼십 (thirty), so knowing 오 and 십 gives you 오십 for free.