Presumably, the square and circle overlap, but this may happen at up to eight places, depending on size and location.
What haven't you told us?
What is the probabilith that a point on a circle is also a point on a square?
2 answers
If the square is inscribed in the circle then it is the radio of the square area to the circle area times100
100 S^2/ (pi R^2)
100 S^2/ (pi R^2)