2019 Mock AMC 10B Problems/Problem 11

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Note that the area of square ABCD is 100. We can break the large square into 20 right triangles, all of which are congruent. Thus, the area of one triangle is 5, and because the smaller square is composed of 4 of these triangles, the area of the smaller square is 20, or $\boxed{\textbf B}$