이십
isip · TOPIK 1
twenty
이십 = 이 (two, 二) + 십 (ten, 十): two tens make twenty
Sino-Korean numbers stack like multiplication: 이 (two) in front of 십 (ten) means two times ten. The same pattern gives 삼십 (thirty) and 사십 (forty). Both syllables come straight from the hanja 二十.