40% of the 12 to 20 million illegal aliens in this country came here legally and overstayed their visa. Securing the border only solves part of the problem.
Now look at what you just said. You quoted an exact percentage, and then had no idea how many folks to apply it to. 12 or 20 million? My point is this: that 40% figure is dubious, very dubious. The biggest part of the problem is due to those who crossed the border illegally and the way to stop the biggest bleeding is to stop that.