十 ten 10 십
Notice that 十 has both a horizontal and vertical stroke.
How would you write 20? 二十 (2 tens)
How about thirty? 三十
Well, isn’t that clever?
Note one more thing. This character 十 has two strokes. So does the character 二. One of the ways to think about Hanja is how many lines does it take to draw the character.