Hi, I can tell you the itinary scheduled time but I do need to know which station you step in. Or you can have a look on the french RATP web site and check timetables. Meantime , there are lot of works on going for the RER B line , so it can become a real burden to go trough. The RATP network web site is attached onto my site: memorablevisit, so hope this helps, and feel free to get in touch with me for advices.With your case last options stressless would be to take a cab from Gare du Nord (waiting queue close to west wing exit (Eurostar) Regards

Niot sure I understand what you need. We are taking metro line 5 in the direction of Bobigny. Getting off this metro at Gare du Nord. We will transfer here. We need to get on the RER B to go to the airport (CDg).

What type of work is going on that you mention? Is this work on the RER B at Gare du Nord? Thank you.

The Metro and RER stops are adjacent to each other in the southeast corner of Gare du Nord, as you can see on this map:
http://goeurope.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=goeurope&cdn=travel&tm=25&gps=355_8_1023_1&f=00&tt=3&bt=0&bts=0&zu=http%3A//mappingeurope.com/france/paris-gare-du-nord-map.htm
The RATP authority, which covers both suburban trains and Metro, estimates the walk at two minutes although strangers navigating through crowds of commuters will probably take more time.
The RATP site has many useful functions, once you figure out its mechanics. Here is the section, in English (sometimes), where you can peg in your destination to find the quickest route from your departure: http://www.ratp.fr/en/ratp/r_61595/finding-your-way-in-paris/
Click on the hazy box labelled Routes.
The blog www.parisbytrain.com offers reassuring advice for public transport. Its page of pictures around Gare du Nord may be helpful:
http://parisbytrain.com/gare-du-nord-photos/

There's just a lot of work in general now on the RER B line, not just at Gare du Nord. I think there is a broken rail line somewhere, that's what the RATP trafic bulletin says. It's nothing you can plan for whenever your trip is. Just don't cut corners short for any trip like that (meaning don't depend on it being correct to the minute, give yourself cushion time).