Simple guess assuming that the answer is an integer: We have 5 copies of equal area with area = 3w, where w is the width of the first box. That means the whole square has area 15w. If the answer is an integer then it means 15w is a perfect square, which requires w = 15u^2 for some integer u. Guessing u = 1, the answer may be 15^2 = 225.