This is what I have so far. My problem is I know that in order for me to draw the circle or the X, I need to compare the mouseX and  mouseY to the right, left, top and bottom boundaries of the tiles. I am not sure how to figure that out. did I set up my game board incorrectly? I am not allowed to use anything complex like matrices and such that is why this is really basic. Can someone please take some time to help me out, I am really desperate:
void setup() {
  int width  = 400;
  int height = 400;
size(width,height);
}
void draw() {
   background(0);

   for(int i =0;i<width;i+=10){
       for (int j=0;j<height;j+=10){
                    
            float my_distance = sqrt((i-mouseX)*(i-mouseX) + (j-mouseY)*(j-mouseY));
            float dot_color   = (my_distance*250)/(width/2);
            stroke(dot_color);
            fill(dot_color);
            point(i,j);
       }
   }
int P1 = 50;
int P2 = 100;
int P3 = 150;
int P4 = 200;

stroke(0);

rect(P2,P2,P1,P1);
if(mousePressed == true){
   if ((mouseX>P2)&&(mouseX<P4)&&(mouseY>0)&&(mouseY<P2)){
     stroke(250);
     fill(150);
     ellipse(150,150,10,10);
   }
}
rect(P4,P4,P1,P1);
rect(P4,P2,P1,P1);
rect(P2,P4,P1,P1);
rect(P3,P3,P1,P1);

fill(0);

rect(P3,P2,P1,P1);
rect(P2,P3,P1,P1);
rect(P3,P4,P1,P1);
rect(P4,P3,P1,P1);

}

Thank you so much in advance...