Here's a really fun game that has the same goal I had in college - to reach about 67%.  The instructions on how to play are below.  See how well you can do in the 5 minutes you have to play.

Instructions:

    * Mouse down to create a circle.
    * If a bouncing ball hits the circle, you'll lose a life.
    * Release the mouse to place the circle.
    * This version is time-locked and will go dead after 5 minutes of play.  

Tips:

    * Watch one spot and wait for it to clear out.
    * All filler balls weigh the same, which means placing small balls on top of big balls can cause a shift in the balance. This is especially useful if you can get a large ball to "push" the bouncing balls towards one side of the screen.
    * Be patient! There's no penalty for running out of time.
    * If possible, avoid the top right corner. If a bouncing ball gets "squished" it will reappear here.
    * Try dropping a ball from near the top of the level. As it travels downward, there will be some null space from where it pushes the bouncing balls out of the way. This is a great place to put more balls!
    * Dropping balls also makes it possible to capture bouncing balls in little cracks, which gives you more space to use.