Out of 100 alerts that the detector sounds, 1% are erroneous alerts (bags with no explosives). So actual number of bags with no explosives will depend on the total number of bags screened. Number of bags with explosives will be likewise determined.
In its concluding part, the argument switches from talking percentages of alerts to number of alerts sounded.
They are not the same.
Please note that while the question says - the alert will beep erroneously FOR BAGS THAT DONT HAVE EXPLOSIVES (1% CASES) it does not expressly qualifies if it will MISS a beep for BAGS that DO contain explosives. I am assuming that the it will beep FOR ALL BAGS that do contain explosives PLUS it will beep additionally for 1% of bags that DO NOT CONTAIN EXPLOSIVES
Take a concrete example - which has been reverse-engineered from the conditions laid out in the question.
Assume there are a total of 298 bags. Out of these further assume that 98 have explosives.
The detector will erroneously beep for 1% of 200 bags with no explosives = 2 beeps.
remaining 98 beeps will correspond to bags with explosives.
Thus we see a case where out of 100 alerts - 98 and not 99 bags actually had explosives.
A certain airport security scanner designed to detect explosives in luggage will alert the scanner’s operator whenever the piece of luggage passing under the scanner contains an explosive. The scanner will erroneously alert the operator for only one percent of the pieces of luggage that contain no explosives. Thus in ninety-nine out of a hundred alerts explosives will actually be present.
The reasoning in the argument is flawed because the argument
(A) ignores the possibility of the scanner’s failing to signal an alert when the luggage does contain an explosive
(B) draws a general conclusion about reliability on the basis of a sample that is likely to be biased
(C) ignores the possibility of human error on the part of the scanner’s operator once the scanner has alerted him or her
(D) fails to acknowledge the possibility that the scanner will not be equally sensitive to all kinds of explosives
(E) substitutes one group for a different group in the statement of a percentage
OA comes later