2019 Mock AMC 10B Problems/Problem 4

To make use of the A=rs formula, we can put four of the isosceles right triangles together to form a rhombus, with a circle circumscribed into it. The area of the rhombus would be the product of the diagonals, divided by 2, or 80 squared/2, which yields 3200. Therefore, 3200=40√2(r). R would equal 3200/40√2, or 40√2. 40√2 in decimal notation would be approximately 28.28, giving a final answer of (C), 28.