Checking the Math of '21' The No. 1 movie at the box office for the last two weekends has been 21, which depicts a group of Massachusetts Institute of Technology students who decide to try to relieve some tuition pressures by hitting the tables in Las Vegas. "Math guy" Keith Devlin talks about the movie.
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Checking the Math of '21'

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Checking the Math of '21'

Checking the Math of '21'

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This is Weekend Edition from NPR News. I'm Scott Simon. Coming up, a paper mill worker publishes his first book.

But first paying for college has never seem more daunting. The number one box office movie the last two weekends, "21," dramatizes a group of MIT students who decide to try to relieve some tuition pressures by hitting the gaming tables in Las Vegas. Now they have a system imparted by their professor played by Kevin Spacey.

(Soundbite of movie "21")

Mr. KEVIN SPACEY (Actor): (As Micky Rosa) We have a total of 76 cards that came out of the deck. Twenty-three were high cards with a value of minus one, 17 were neutral with no value at all and the rest were low cards with a value of plus one. How could you lose the count?

Mr. JIM STURGESS (Actor): (As Ben Campbell) Plus 13, the count.

Mr. SPACEY: (As Micky Rosa): Yes. Pull up a chair.

SIMON: That's Kevin Spacey and Jim Sturgess. We've asked our own card shark, our math guy Keith Devlin, to pull up his chair. Keith, how are you?

KEITH DEVLIN: Very well, thank you.

SIMON: You know, some people go to the movies for the stars, we sent you for the math. Does it add up?

DEVLIN: The math odds are - the math part thrills in the movie, they actually get it right, and it's all based on true cases in the mid-1990s. There were actually I think at one stage a hundred or so teams of MIT students that would regularly go out to Vegas to play the casinos. They took a method for beating the casinos at blackjack that had been invented way back in the early 1960s by a then-MIT professor of mathematics, Edward Thorp. They then in the course of the '80s and '90s refined that method so that it was actually a sure-fire thing. They were no longer gambling if they played that method. They were guaranteed that over many hours of play, and the movie doesn't make that part clear, you will make a very, very good profit.

SIMON: It's kind of made to look easy in this film, I gather.

DEVLIN: The method itself is extremely simple. It's not a matter of counting numbers of cards or adding up the totals. As the movie makes very clear, you have this simple number that you carry in your head. You - which ever way you set it up, you add one if it's lots of low cards. When the low cards come out you subtract one, when high cards come out - I mean the point is that because of the way blackjack is played in casinos, when the proportion of high cards, nines, tens, face cards, is high, the player has an edge over the casino, so that if you play them many, many times, you can win.

So Thorp introduced this idea of a single number that you just adjust with every play by either adding one or subtracting one or leaving it as it is, and carrying that in your mind and then when the number tells you there's a large number of face cards in the deck, you start to bet increasing amounts of money.

The problem is although the arithmetic is simple, you have to do it in a noisy casino over many, many hours when you're getting tired and knowing that there are cameras in the ceiling that are looking at you and if they detect that you concentrating on something other than the game, two very large gentleman will base - they won't have baseball bats, but two very large gentlemen will come along and escort you out of the establishment.

SIMON: The process you describe - is that essentially what very gifted blackjack players do?

DEVLIN: You certainly need to be very good at instant arithmetic and have a good focus to be able to carry that count. I tried it once as an experiment, not in a casino…

SIMON: I was just working my way up to that question. All right, yeah.

DEVLIN: And it's extremely easy to lose track of that count. Most of the people that have tried this, and when Thorp's book came out in the early '60, thousands of people tried it and hardly any of them were successful because it's incredibly difficult to carry that number and remember it. And then that clip you played from the movie is accurate in that it shows that it takes a very unusual individual to make it look as though you are actually just being lucky whereas in fact, you are doing mathematics all the time on every single hand.

SIMON: Keith, thanks so much. Nice talking to you.

DEVLIN: Okay. My pleasure, Scott.

SIMON: Keith Devlin, our math guy, speaking to us from Stanford University.

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