Probability of holding 3 different posts in a committee

[desmond iking]

Homework Statement

A , B and C are from a class of 40 students. Three students are selected to be a monitor, assistant monitor and treasurer.
a) What's the probability of A to be monitor, B to be assistant monitor , C to be treasurer

my ans is (1/(40 x 39 x 38))=1/59280

b. What's the probability of A , B and C are selected to hold three post?

can someone help me on part b plaese?

my working is 40C3=9880

but the ans is 1/9880

Homework Equations

The Attempt at a Solution

 

[Ray Vickson]

Homework Statement

A , B and C are from a class of 40 students. Three students are selected to be a monitor, assistant monitor and treasurer.
a) What's the probability of A to be monitor, B to be assistant monitor , C to be treasurer

my ans is (1/(40 x 39 x 38))=1/59280

b. What's the probability of A , B and C are selected to hold three post?

can someone help me on part b plaese?

my working is 40C3=9880

but the ans is 1/9880

Homework Equations

The Attempt at a Solution

Presumably you wrote down 40C3=9880 for some reason. What is that reason?

 

[desmond iking]

Presumably you wrote down 40C3=9880 for some reason. What is that reason?

choose 3 person form 40 person for the 3 posts

 

[Ray Vickson]

choose 3 person form 40 person for the 3 posts

OK, so now what should you do?

Personally, I would start from the solution to (a) and think about it a bit more: your solution to (a) is for the ordering (monitor,assistant monitor, treasurer) = (A,B,C). However there are other allowed selections, such as (A,C,B) and several others. They all have probabilities of their own, and their total is what you are after.