order of operations (PEMDAS):
21 - 18 / 3 2
let's add some parenthesese to make it easier to see:
21 - (18 / 3 2)
multiplication and division are associative
also: 18 / 3 = 18 (1/3)
21 - (18 (1/3) 2)
21 - (62) or 21 - (36 (1/3)) or 21 - (18 (2/3))
21 - 12
9
