I've covered this whole square.
with these 2 tiles:
Now if I took away one red piece and add in one green piece, is it still posible to cover the whole square?
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Consider the figure below.
Each 2 × 2 square covers exactly 1 blue square. Each 1 × 4 tetromino covers an even number of blue squares. There are 6 4 red squares. If we used x number of green tiles and the red tiles covered y number of blue squares , then x + y = 6 4 where y is an even, so x is even too. But if we use one more green tile, then x will be odd, so then the covering is not possible.