okay guys...have chnaged my hotel booking from Momntmartre to "IBIS Bastille fauborg st.antoine" which is on the trousseau Rue in th 11th district and more central i guess.
Any tips on how to reach it from ORLY airport by subway .
Also is it a good idea to buy a carnet of 10 tickets for two people ? My husband and i will be staying two days in Paris and then taking the EUROSTAR to London.

Your metro stop is Ledru-Rollin. My guess is that you would take the Orly shuttle to the RER and the RER to the Chatelet to transfer to the metro. Your RER ticket is good for the metro. A carnet is a good idea. That's only five tickets per person for two days.

As I wrote on TA, I think it would be easier to just walk from Bastille.

I think you will find the subway is called a metro .

You will be MUCH happier there. We have even stayed there, but I can't remember the Metro stop. It is about 6 blocks from Bastille.
A taxi from Orly would only be about 20E. I seem to recall you have never been to Paris. Chatelet is a nightmare to walk/change. Why not start your trip with a treat and be delivered to the door of your hotel.