[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]

How many ways can six indistinguishable letters be

put in three mail boxes? Hint: One representation of this is given by a sequence [math]|[/math]LL[math]|[/math]L[math]|[/math]LLL[math]|[/math] where the [math]|[/math]'s represent the partitions for the boxes and the L's the letters. Any possible way can be so described. Note that we need two bars at the ends and the remaining two bars and the six L's can be put in any order.