15. In the 6 circles below, fill in $1,2,3,4,5,6$, using each number only once, so that the sum of the three numbers on each side is equal.
【Analysis】Let the 3 vertices be $a, b, c, 1+2+3+4+5+6+a+b+c=21+a+b+c$ is the sum of the 3 edges, $a+b+c$ is a multiple of 3. When $a+b+c=1+2+3=6$, the sum of each edge is $(21+6) \div 3=9$. By rotating the above two numbers, 4 more results can be obtained.