Kathy understood exactly what I meant.
Two carnets is twenty one way tickets.
That is ten return trips. One ticket to go some place. One ticket to go back.
I think the ND is a good deal for people who intend to use transport fairly often.
Between my partner and I we used two carnets in 8 days last summer, and I think we had a few tickets left over.
Almost any site a tourist wants to see is within zones 1-2.
Getting a zone 1-5 ticket does add a lot of value if planning on Versilles and Disneyland, And plan on airport commute.
Airport to Paris- 20 euros return
Paris to Disneyland- 14 euros return
Paris to Versailles- 8 euros return
Prices approx,, a few centimes more here and there, but I underestimated so you do get the picture.
And not for everyone. Dates must work out , and you should likely not be a rabid walker like me.