The Peanuts cartoon (page 535) expresses a common feeling regarding chain letters. Consider the total number of letters sent after a particular mailing:

\[\begin{array}{lr}\text { 1st mailing: } & 6 \\\text { 2nd mailing: } & 6+36=42 \\\text { 3rd mailing: } & 6+36+216=258\end{array}\]

Determine the total number of letters sent in five mailings of the chain letter.