Angular separation between moons

[Scott Yuill]

Please help I am not sure of how to work this out

The angular separation between Styx and Nix 28 days after they are in line with each other on the same side of Pluto.

Mass of Nix = 5*10^16 Kg
Orbital Radius of Nix = 48675 Km
Orbital period of Styx = 20 Earth days

 

[haruspex]

Through what angle about Pluto will Styx have gone in 28 days?
How will you calculate the angle for Nix?