In a classroom the students are 12 boys and 6 girls If one student is selected at random find the probability that the student is a girl?

If there are 12 boys and 6 girls, the total number of students would be 18.

You are looking for the probability of selecting a girl from the class:

6/18

Just reduce the fraction to its lowest form

6/18 = 2/6 = 1/3

Your answer would be 1/3.