Probability of 2 or more students from a group of 5 being on the same team of 6 in a class of 36
To clarify the title: There is a class of 36 which is to be divided into 6 teams of 6. There are 5 specific students of interest, and I want to figure out the probability of 2 or more of those 5 being on the same team.
I know there are other questions like this, but I’ve tried to use multiple methods to do this and keep getting different answers so I’m clearly doing something wrong.
I started by trying this:
$$\frac{6*5*4*3*2}{36*35*34*33*32} + \frac{6*5*4*3}{36*35*34*33} + \frac{6*5*4}{36*35*34} + \frac{6*5}{36*35}$$
I got around 19.35%.
Then, I tried this because I thought maybe the first team member can go anywhere so the 6/36 is unnecessary:
$$\frac{5*4*3*2}{35*34*33*32} + \frac{5*4*3}{35*34*33} + \frac{5*4}{35*34} + \frac{5}{35}$$
I got 16.13%.
Then, I tried to modify a method I saw in a similar answer like so:
$$\sum_{n=2}^{5} \frac{_{36-n}C_{6-n}}{_{36}C_{6}}$$
I got 2.69%.
Yeah, I really don't remember how this works...
It appears that you want the probability that the specified students are not all on different teams. This is simply the compliment $(1-p)$ of the probability that they are all on different teams.
$$P(\text{all different teams})=\frac{36}{36}\times\frac{30}{35}\times\frac{24}{34}\times\frac{18}{33}\times\frac{12}{32}=\frac{162}{1309}\approx12.4\%$$
$$P(\text{not all different teams})=1-\frac{162}{1309}=\frac{1147}{1309}\approx87.6\%$$