A certain one-day seminar consisted of a morning seminar and an afternoon seminar. If each of the 128 people attending the seminar attended at least one of the two sessions, how many of the people attended the morning session only?
1) 3/4 of the people attended both sessions
2) 7/8 of the people attended the evening session
The Answer is B
Option 1 alone is not giving any info about the remaining 1/4 of the people.
Option 2 alone gives the combined information about those who have attended both sessions or only evening sessions. Hence it can easily be deduced using 2 alone how many of the people have attended the morning session only.
This will be more clear if you try to use Venn Diagrams.