Check this reply for some other forum.

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To me, this is an overlapping sets problem best solved with a two-circle Venn Diagram. Circle #1 is first day attendants, circle #2 is second day attendants, the overlap is attendants for both days, and Neither is the space outside the diagram.

The information from the original problem plus statement (1) give you the first circle (900). Insufficient to solve for Neither.
The information from the original problem plus statement (2) give you the second circle (800). Also Insufficient to solve for Neither.

Together, you still have no idea how many individuals addended Both days. Since the proper formula for such a situation is
Total = Group 1 + Group 2 - Both + Neither, in our case we have
1000 = 900 + 800 - Both + Neither.
Since we are still left with two variables (B and N), the correct answer is E.

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1) A total of 1000 people registered for the two day seminar
----> but it does not say how many attended on 2nd day...noyt sufficient
2) Of those registered, 80 percent attended the seminar on the second day
--- no infomation on those who attended on 1st day...or who does not.

Consider 1 and 2:

Since, the 80% of 2nd day can contains all from the 90% of 1st day or 70% of the fist day + 10 % remaining who did not attended on 1st day....so, nothing can be concluded...Hence E.