Rectangle with corners (1, 1), (7, 1), (7, 5) and (1, 5): at which point do its diagonals cross?

The diagonals of a rectangle bisect each other, so they cross at the centre: halfway across (1 to 7 gives 4) and halfway up (1 to 5 gives 3), i.e. (4, 3). (8, 6) adds the coordinates without halving.