Chapter 3 - Section 3-5 - Discrete Uniform Distribution - Exercises - Page 79: 3-86

The probability that it belongs to one of the 300 million people in the United States $=\frac{300000000}{1000000000}=0.3$

let X is the random variable (nine-digit numbers), this means that $f(x)=\frac{1}{1000000000}, x=0,1,2,..,999999999$, then, the probability that it belongs to one of the 300 million people in the United States $=\frac{300000000}{1000000000}=0.3$