About eleven-twelveths of a state park is covered by trees, one-eighteenth is covered by a lake, and the rest is covered by open fields. What part of the state park is open fields?

Answer 1

11/12 = 33/36
1/18 = 2/36

(36/36) - (33/36) - (2/36) = ?

Answer 2

12 = 2*2*3
18 = 2*3*3
lowest common multiple = 2*2*3*3 = 4*9 = 36

11/12 = 33/36
1/18 = 2/36

so 35/36 is used
and
1/36 remains open fields