The class starts out with 26 students, of whom 12 are girls and, perhaps unlike Mr Morris, are sure of it.
The probability of the first random choice being a girl is 12/26. If successful, there are now 25 students left, of whom 11 are girls. The probability of a girl on the 2nd random choice is 11/25. The probability that BOTH random choices are successful is (12/26) x (11/25). That's (132/650), or about 20.31% (rounded).
