Four angels sat on the Christmas tree amidst other ornaments. Two had blue halos and two – yellow. However, none of them could see above his head. Angel A sat on the top branch and could see the angels B and C, who sat below him. Angel B, could see angel C who sat on the lower branch. And angel D stood at the base of the tree obscured from view by a thicket of branches, so no one could see him and he could not see anyone either.
Which one of them could be the first to guess the color of his halo and speak it out loud for all other angels to hear?
Case 1: Angels B and C have same color(either both blue or yellow) Then A can be sure of his color.
Case 2: Angles B and C have different colors.
B after waiting for some time will realise that A has not spoken out the color yet ,hence B and C must have different colors. B will look at C's color and announce his color opposite to C's color.