A pack on a day

While seeing a 56 y/o male:

Me: How much do you smoke?

Patient: Not much, I'll only buy a pack on a day.

Me: A pack a day, that's a lot, that's 20 cigarettes a day. That means you're smoking almost 1 an hour.

Patient: No I don't.

Me: Aren't there 20 cigarettes in a pack?

Patient: Yes.

Me: So you smoke 20 cigarettes a day?

Patient: No, when I buy a pack on a day it will usually last me 4-5 days before I have to buy another pack.

Me: Oh, I see. So you're smoking 4-5 cigarettes a day...hopefully you can continue to get even lower than that.

Actually, my initial thought was, "oops, my bad, ...sorry!"