We know that the sandbox is square, and that the area is 160 sq ft. Since A = s^2 for a square, let's work backwards and find s if A = 160 sq ft = s^2.
s^2 = 160 = 16(10). We do this because 16 is the largest perfect square factor of 160.
Then the length of one side of the square is approx. s = 4√10 ft (12.65 ft)