By a simple observation we can see that opposite pies, when added give a total of 21.
Following that logic we formulate this equation for the unknown pie, where we need to solve for x:
15 + x = 21
we subtract 15 from both sides and get x = 21-15 = 6
Therefore, the answer is 6
RE: Maths Brain Teasers 66:: Can You Complete The Pie???