indecipherable vs unsolvable

indecipherable

adj

  • Not decipherable; impossible to decode, read, understand or comprehend. 

  • Indistinguishable 

unsolvable

adj

  • Not solvable. 

noun

  • A problem that cannot be solved. 

How often have the words indecipherable and unsolvable occurred in a corpus of books? (source: Google Ngram Viewer )