Put points A, B, C, and D on a line consecutively, so that AB = BC = CD = 6 in. Find the distance between points M and N, the midpoints of segments AB and CD respectively.

AD = 6·3 = 18 in.
AM = AB/2 = 6/2 = 3 in.
DN = CD/2 = 6/2 = 3in.
MN = AD - (AM+DN) = 18 - (3+3) = 12 in.