As they are as likely to wear skirts as trousers, this is 0.5.
* , or the probability of the student wearing trousers given that the student is a boy. This is given as 1.
* , or the probability of a (randomly selected) student wearing trousers regardless of any other information. Since (via the law of total probability), this is .

Given all this information, the posterior probability of the observer having spotted a girl given that the observed student is wearing trousers