Thanks for your reply. I can use the ReR ticket you quoted for the train and metro ? Or I need separate tickets for metro. If I need separate tickets How much will the metro tickets cost please ??

Ben Lam⋅ Staff⋅ 2 days ago

RER A from Marne La Vallee to Nation station, transfer to Metro line 6 direction Charles de Gaulle-Etoile until Montparnasse Bienvenue station.  Walk to Gare Montparnasse from there.
 
Trains are frequent.  Every 10 minutes roughly.
 
Buying single use RER tickets at Marne La Vallee will be cheapest option. 7.60€ for each adult and 3.80€ for any child 9 and under.