difference(){
union()  {
cube([10,20,40]);
translate([10,0,0])cube([10,20,20]);
 translate([20,0,0])cube([10,20,20]);
 translate([30,0,0])cube([10,20,40]);
  }

   translate([-1,10,30])rotate([0,90,0])cylinder(h=45, r=2, $fn=20);
}