Skyscraper Windows Problem (doc | pdf)

A building is 12 stories high and is covered entirely by windows on all four sides.  Each floor has 38 windows on it.  Once a year, all the windows are washed.  The cost for washing the windows is $2.00 for each first-floor window, $2.50 for each second-floor window, $3.00 for each third-floor window, and soon.  How much will it cost to wash the windows of this building?  What if the building is 30 stories tall?  n stories tall? (paraphrased from Driscoll, 1999, p. 70)