probability - customer who bought this also bought this [closed]

How to calculate probability that a customer bought this item also bought this other item?

I tried using google to no avail. It seems very simple, but I still am confused.

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closed as not a real question by froggie, Austin Mohr, Micah, Alexander Gruber♦, Davide GiraudoDec 14 '12 at 10:33

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center. If this question can be reworded to fit the rules in the help center, please edit the question.

What is the application, and what is given? –  apnorton Dec 14 '12 at 2:28
@anorton All the items all the customers bought. Like Customer1->X1, Customer2->X2, Customer1->X3 and so on where X1, X2 AND X3 are products. –  user1831003 Dec 14 '12 at 2:32

A simple way to do this would be to count all the people who bought product $X$, then count all the people who bought product $X$ and $Y$. This (somewhat) tells you the probability that someone who buys $X$ will buy $Y$.