How can I travel from CDG to Gare Montparnasse?

To travel from Charles de Gaulle Airport (CDG) to Gare Montparnasse in Paris, you can take the RER B train from the airport to Denfert-Rochereau station, then transfer to the RER line 6 to Montparnasse-Bienvenue station. The journey takes about 1 hour and costs around 10 euros.