IMP3 POW 1...HELP PLEASE?

okay so for those of you that are in imp of any kind, you know that POWs are one of the most difficult homeworks you get, well in my imp3 cl we were igned a POW where there are rats on a ship and we have to find out how they multiply if 6 are born from each litter and when they are 120 days old they have thier first kids. of the 6 3 are ALWAYS female and 3 are ALWAYS male....please please please help me, i was able to get the right answer to day 160 but I cant figure out where to go from here. help would be MUCH APPRECIATED!