Of 55 Employees of a Store Located in Downtown Boston

Of 55 employees of a store located in downtown Boston, 18 people take the subway to work, 15 take the bus, and 6 take both the subway and the bus.
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How many employees take the subway or the bus to work? __________
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How many employees take only the bus to work? __________
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How many employees take either the bus or the subway to work? __________
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How many employees get to work by some other means? __________