Board Thread:Character Discussion/@comment-25176701-20150119013621/@comment-26247132-20150701194735

Sending the 100 to earth gave them an extra month of oxygen. Culling 209 people the next day would have given them 2 more months. Culling 320 people 11 days later (because they waited, meant more people had to die) gave them 2 more months.

3 months + 1 month = 4 months

4 months + 2 months = 6 months

Otherwise, 3 months of oxygen left plus 3 months wasted on an illegal spacewalk, you get:

3 months + 3 months = 6 months