A divergent series from Futurama

(1) The situation you describe (2 copies at 60% the mass) is a divergent series.

(2) The sum in the picture linked above is also divergent -- it's harmonic.

(3) The sum in the picture does not represent the situation you describe. For that, the sum would look like

$$M = \sum_{n=0}^\infty 2^n M_0 (0.6)^n$$

which is a divergent geometric series.

(4) The head writer on Futurama studied physics at Harvard and CS at Berkeley, has published math papers, and earlier episodes of Futurama have featured much more sophisticated math than this, so it's likely that the equation displayed above accurately represents the situation described in the show. I'll have to try to catch a rerun and see exactly what Farnsworth says.