Spelling mistakes in the text are either "nonword errors" or "word errors." A nonword error produces the string of letters that isn't a word, such as "the" typed as "teh." Word errors produce wrong word, such as "loose" typed as "lose." Nonword errors make up 25% of all errors. A human proofreader will catch 80% of nonword errors and 50% of word errors.
What percent of errors will the proofreader catch?