Consider the +X axis and the +Y axis to be the corner. The table has radius r which puts the center of the circle at (r,r) and makes the circle tangent to both axis. The equation of the circle (table's perimeter) is
This leads to
Using x = 10, y = 5 we get the solutions 25 and 5. The former is the radius of the table. Its diameter is 50 cm.