List the measurements of three different rectangles that each has a perimeter of 20 units.

1. Start with the perimeter formula for a rectangle: P = 2l + 2w.
2. Given that the perimeter P is 20 units, set up the equation: 2l + 2w = 20.
3. Divide the entire equation by 2: l + w = 10.
4. Find pairs of integer values (l, w) that satisfy the equation l + w = 10:
- If l = 1, then w = 9: l + w = 1 + 9 = 10
- If l = 2, then w = 8: l + w = 2 + 8 = 10
- If l = 3, then w = 7: l + w = 3 + 7 = 10

Each of these pairs represents measurements of a rectangle with a perimeter of 20 units:
- (1, 9)
- (2, 8)
- (3, 7)