Three students are selected at random from a group of 6 boys and 4 girls. What is the probability that at least 2 girls are selected?

There are $C(10,3)=120$ possible groups. Exactly 2 girls: $C(4,2)\times C(6,1)=6\times 6=36$ groups. Exactly 3 girls: $C(4,3)=4$ groups. 'At least 2' adds these two cases, giving 40 groups and a probability of 40/120 = 1/3. Answering 36/120 = 3/10 counts only the 'exactly two' case.