Bidisha_800-
We're asked to determine if the mode of a set is equal to the mean. The mode is the most frequent member of the set (in this case it is unique - which means there is only one mode). The mean is the average of all the members of the set.
(2) Tells us that three of the five set members are the same. Obviously, this value will also be the mode (as the most frequent member).
So, let's suppose (as cramya wrote) that the set is...
{1,2,2,2,3}
This set meets the requirement of a unique mode = 2. It also meets the requirement of (2) in that three of the five are the same {2,2,2}.
The average of this set is (1+2+2+2+3)/5 = 10/5 = 2.
So, the mean and mode are equal. BUT, are they equal for all possible sets under condition (2)?
Let's suppose (thanks to cramya again) that the set is...
{0,0,0,2,3}
Mode = 0. Three of five are the same {0,0,0}.
The average of this set is (0+0+0+2+3)/5 = 5/5 = 1.
So, the mean and mode are NOT equal. So, we cannot determine conclusively that the mode will equal the mean when three members of the set are the same.
(2) is Insufficient.
Hope that helps.