Find the total ways that people can be seated with no restrictions;
6! ways but you know that marcia and jan will not sit together.
Write out this, as: G, (M, J), B, P <-- M and J (Marcia and Jan are grouped together to find the ways they will be sat together, so we can subtract this away).
In this combination, considering M and J as one, you have 5! ways. Then you see that M and J can switch position, so there would be added 2! ways.
In total, possibility of J and M to sit together is 5!2!
6!-5!2!
=720-240
= 480 possible ways of seating the friends so that Jan and Marcia are not together.
Hope I helped :)
