The right-side rearview mirror of Mike's car says that objects in the mirror are closer than they appear. Mike decides to do an experiment to determine the focal length of this mirror. He holds a plane mirror next to the rearview mirror and views an object that is 167 cm away from each mirror. The object appears 3.23 cm wide in the plane mirror, but only 1.78 cm wide in the rearview mirror. What is the focal length of the rearview mirror?