Answer:
We do not have the total number of students or volunteers.
I will put variables (and then random numbers) and solve it, then you can put the numbers that you need in the equations.
Suppose that the class has Y students, and X of these students have volunteered.
The probability of picking at random a student that has volunteered is equal to the number of students that had volunteered divided by the total amount of students:
p1 = X/Y
now, we must choose another student, but now the volunteers are X - 1, and the total number of students is also Y - 1 (because we already took one student)
now, the probability of selecting other will be:
p2 = (X- 1)/(Y - 1)
Then, the joint probability of both events is equal to the product of both probabilities:
P = p1*p2 = (X/Y)*((X-1)/(Y -1))
Suppose that there are 20 students, and 10 students that have volunteered, then the equations are:
P = (10/20)*(9/19) = 0.237
