How do you calculate correlation coefficient?

A hifi store kept track of the number of advertisements it put in the local newspaper vs the number of stereos they sold each week. They found that they were able to find a correlation coefficient that would explain the relationship between number of advertisements and number of stereos sold. It was found that there was a general trend that would say that the more advertisements put in the paper would also make the store sell more stereo systems.

For example if one advertisement was put in the paper six systems were sold, if two advertisements were put in the paper eight systems were sold, and if three advertisements were out twelve systems were sold. A definite trend was shown to say that more advertisements did indeed create more sales.