Spell-checking software catches "nonword errors", which result in a string of letters that is not a word, as when "the" is typed as "teh". When undergraduates are asked to write a 250-word essay (without spell-checking), the number of Y nonword errors in a randomly selected essay has the following probability distribution:

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(a) Write the event "one nonword error" in terms of Y. Then find its probability.

(b) What's the probability that a randomly selected essay has at least two nonword errors?