A good answer might be:

There are many interesting ways to fill in the blank. My choice is:

int count = 1;
while ( count <= 10 )
  circ.setRadius( (100/10)*count );
  count = count + 1;

Since the width (and height) of the drawing area is 200, circles can have a radius of up to 100. For ten circles, that maximum radius is divided by ten to give the size increase in each radius.

Concentric Circles Applet

Here is what the applet draws:


It would be nice to do something with color. What two things should be done to the class for drawing colored circles?