Question 20
Consider a $5\times4$ grid, in which a $2\times2$ black square has already been placed in the top-left corner, as shown in the diagram below on the left. You are to place five $1\times2$ rectangles onto the remaining unoccupied cells. The rectangles must all be placed horizontally (no rotations allowed), aligned along the grid lines, and cannot overlap with each other or be placed over any of the black squares. One valid arrangement is shown below on the right. How many such arrangements, including the given valid arrangement, are possible?
