Answer:
(a) 0.2824
(b) 0.0002441
Explanation:
Assuming that the population is large enough to behave as a binomial distribution (between whites and non-whites), the probability of 12 out of 12 juros being white is:

Where p(w) is the proportion of the population that is white:
(a) for p = 0.90

The probability is 0.2824.
(b) for p = 0.50

The probability is 0.0002441.