In four schools \( B_1, B_2, B_3, B_4 \), the number of students is given as follows:
\[
B_1 = 12, \quad B_2 = 20, \quad B_3 = 13, \quad B_4 = 17
\]
A student is selected at random from any of the schools. The probability that the student is from school \( B_2 \) is: