[math] \newcommand{\NA}{{\rm NA}} \newcommand{\mat}[1]{{\bf#1}} \newcommand{\exref}[1]{\ref{##1}} \newcommand{\secstoprocess}{\all} \newcommand{\NA}{{\rm NA}} \newcommand{\mathds}{\mathbb}[/math]

Our calendar has a 400-year cycle. B. H. Brown noticed that the

number of times the thirteenth of the month falls on each of the days of the week in the 4800 months of a cycle is as follows:

Day	Frequency
Sunday	687
Monday	685
Tuesday	685
Wednesday	687
Thursday	684
Friday	688
Saturday	684

From this he deduced that the thirteenth was more likely to fall on Friday than on any other day. Explain what he meant by this.