At the beginning, the chances that the teacher will choose a student who has forgotten his lunch are 2/10, or 1/5.
If the teacher now chooses another student from the remaining 9 students, the chances of his choosing a student who has forgotten his lunch are 1/9, since one of the forgetful students has already been removed from the original group of 10 students, and there are only 9 in the group remaining.
The probability that both students forgot lunch is (1/5)(1/9), or 1/45.
