He is selecting a random student from the total number of all 28, so that's our denominator. It doesn't specify boy or girl, so it could be either. The total number of students wearing non-white shirts is 19. 19/28
This probability is the same as 1-(two different suited cards). Your first card can be any of the 52 cards. Your second card must be any of the 39 different suited remaining 51 cards. So the answer is 1 - (52/52)(39-51). Note: I assumed there was no replacement.
