CareerCup

Pick 1 ball from mixed labeled box - since the label is mixed, it means that the real label is either Black or White. So, if you get a white ball then the correct label should be White.
After this, remaining boxes left are labels White and Black. So, the only possibility is the real label of White box is black..and the remaining black box is real mixed color.

First of all, there is no mixed label.
Secondly, even if there IS a mixed label, and a white ball is drawn out of it,there is a possibility that the remaining box labelled "White" can be a mixture. So to eliminate that, you should pick two more balls. Given the case, that the mixture is equally distributed b/w white and black (which is not), then a min of 3 balls have to be picked up to eliminate if that box is a mixture or not. So i think the answer is 4. (minimum 4 provided the mixture is equally distributed)

My Apologies.. i was wrong...
The problem is very simple in case there is a mixed label.
Because if i pick a white from Mixed Label, there are two possibilities, the box labelled White contains either Black balls or a mixture. But if White label, had a mixture, then Black will be correctly labelled, which is not the problem in hand, (All are wrongly labelled here!!)
Hence 1 pick is sufficient to correctly label 'em all