19. (1985 American High School Mathematics Examination) In a mixed class of boys and girls, a student is selected to represent the class, with each student having an equal chance of being selected, and the probability of a boy being selected is $\frac{2}{3}$ of the probability of a girl being selected. Then the ratio of the number of boys to the total number of students in the class is ( ).
A. $\frac{1}{3}$
B. $\frac{2}{5}$
C. $\frac{1}{2}$
D. $\frac{3}{5}$
E. $\frac{2}{3}$
19. B. Reason: Let the probabilities of a male and a female being selected be $p_{1}, p_{2}$, respectively. According to the problem, $p_{1}=\frac{2}{3} p_{2}$, which means $3 p_{1}=2 p_{2}$,
thus (number of males) : (number of females) $=2: 3$.
Therefore, the proportion of males in the entire class is $\frac{2}{5}$.