The numbers 1, 6, 8, 13, 15, and 20 can be placed in the circle below, each exactly once, so that the sum of each pair of numbers adjacent in the circle is a multiple of seven.
In fact, there is more than one way to arrange the numbers in such a way in the circle. Determine all the different arrangements. Note that we will consider two arrangements to be the same if one can be obtained from the other by a series of reflections and rotations.
Puzze Source: CEMC