Answer:
There is a 14.71% probability of teacher picking all 3 girls.
Explanation:
Initially, there are 18 students, of which 10 are girls. So there is a 10/18 = 5/9 probability that the first student chosen is a girl.
Now, since the first student chosen was a girl, there are 17 students, of which 9 are girls. So there is a 9/17 of the second student chosen being a girl.
Both students picked were girls. Now there are 16 students, of which 8 are girls. So there is a 8/16 = 1/2 probability of the third student chosen in this situation being a girl.
Probability of teacher picking all 3 girls?
We multiply these probabilities:

There is a 14.71% probability of teacher picking all 3 girls.