Among 180 people 27 are managers, 100 are floor assistants and the rest are helpers. Under the rest we mean 180-27-100=53 people.
The event "randomly selected employee is not a floor assistant" is the same as "randomly selected employee is manager or helper". You can count that in total 27+53=80 people form the set of managers and helpers.
Then the probability that randomly selected employee is not a floor assistant is
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