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Where can I find historical P/E ratios for companies? 
The mathematics site, WolframAlpha, provides such data. Here is a link to historic p/e data for Apple. You can chart other companies simply by typing "p/e code" into the search box.  For example, "p/e XOM" will give you historic p/e data for Exxon. A drop-down list box allows you to select a reporting period : 2 years, 5 years, 10 years, all data.  Below the chart you can read the minimum, maximum, and average p/e for the reporting period in addition to the dates on which the minimum and maximum were applicable.