1.
twenty
The number twenty. Formed by combining 二 (two) and 十 (ten), literally meaning 'two tens.'
Twenty has a special reading 二十 specifically when referring to age (turning 20). This is because 20 is traditionally the age of adulthood in Japan. In other contexts, 二十 is used. The Japanese number system is very regular: tens are formed by [number] + 十.