solution vs thank-you note

solution

noun
  • A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances. 

  • An act, plan or other means, used or proposed, to solve a problem. 

  • An answer to a problem. 

  • A product, service or suite thereof, especially software. 

  • The act of dissolving, especially of a solid by a fluid; dissolution. 

verb
  • To treat with a solution. 

thank-you note

How often have the words solution and thank-you note occurred in a corpus of books? (source: Google Ngram Viewer )