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Explanation:
We're told that "given the customer has a carry-on bag". This means we know 100% that the customer has a carry-on bag and we don't need to worry about the other rows. We'll only focus on row 1.
Of the 601 people who had a carry-on, there are 342 people who had a short flight.
This leads to the fraction 342/601
It cannot be reduced because 342 and 601 don't have any factors in common (other than 1).