Find the area of the polygon with vertices of A (0,1), B (0,5), C (4,5), and D (6,1).

20

If you draw that out on a coordinate grid, you get a trapezoid with one base 6 units long, one base 4 units long and a height of 4. Using the formula for area of a trapezoid

h (b1 + b2) / 2

where h is the height, 4, b1 is a base, 6, b2 is the other base, 4, we get

4 (4+6) / 2

which is 4 (10) / 2 or 40/2

which gets us the answer, 20