Only 7.2 percent of the population has O-negative blood A mobile blood center is visited by 20 donors in afternoon Let X denote number of universal donors among them probability that X is at least 2?

• The probability that any given donor is a universal donor is 0.072.
• We need the probability that the number of universal donors in this group of 20 is not zero or one.

Probability of getting zero universal donors: ( 1 - 0.072 )^20 = 0.224367

Probability of getting one such donor: 0.348156 (given by the binomial probability density function: probability of one success in 20 trials with p=0.072)

Total: 0.224367 + 0.348156 = 0.572523, the probability of zero or one donors

But we want 1 - 0.572523 = 0.427477, the probability of getting two or more such donors.

^ stands for 'to the power of'