If I read all this correctly: .Viruses reach networks by requests for communication that are identified as packets.
A security system can detect them if at least one such packet is observed by a network sensor.
Assume that the probability of observing a single packet sent from a new virus is 0.001.
The virus actually sends 150 packets to the network computer.
What is the probability that the virus is detected by the sensor?
There are 150 packets to inspect.
The probability that it detects no virus is: .
Therefore, the probability that the virus is detected
. . (at least once) is: .