MATH PUZZLE: Can you replace...

There is this taxi driver in New York City, and it is nearing the end of his shift but he decides that he will pick up one more person before he turns in for the night. So he stops and pulls over and a nun gets in the car. She tells him where to go and they start off. It is a long drive and the driver keeps looking at the nun through the rear view mirror of his cab.
Now, the nun is getting very upset by this and says, "Um...can I help you??!!"
The taxi driver looks very embarrassed and says, "I'm sorry, it's very embarrassing, I cannot say."
And the nun looks at him and says, "Now, my son, I have been in this business far too long to be disturbed by anything you have to say, so go ahead."
The driver thinks for a minute and says, "Ok, well, for as long as I can remember, I have had the biggest fantasy about kissing a nun."
She looks amused at first then replies, "Well, I think I can help you with that, but first you must promise me two things, the first is that you are a Catholic, the second is that you are single. If you can promise this, I shall give you what you ask for."
And the driver says, "Great!! Sure I'm a single Catholic!!"
So they pull into an alley and the nun crawls into the front seat of the cab and gives the driver an amazing kiss. Porn stars would be envious of this kiss. And they finish up and get back on the road.
Soon the taxi driver starts looking nervous and peering at the nun in the rear view mirror again. Just staring at her, and when the nun asks him why he is staring and he says, "Well, I'm afraid I haven't been completely honest with you. You see, I'm not a catholic, and I'm also married."
The nun smiles and says, "Well, I haven't been honest with you either...My name is Kevin, and I'm going to a Halloween party."