## Help with calculating probability?

I have a strange set of data, it was counting the number of animals entering and leaving a shelter as a 'count' of how many were in the shelter. We were told to make this the only variable and not to note the time at which the animals left or entered. My data was:

0
1
2
1
2
3
4
5
6
7
6

So 8 joined (J) and 2 left (L). There was a total number of 12 cockroaches (some didn't enter/interact with the shelter).
I am trying to calculate the probability of an animal joining/leaving as a function of the total number of cockroaches left under the shelter.
We were total we were to calculate 'P(L|J)'.

With calculating the probability would I do:
P(L)= 8/10 (as there were 10 interactions in total)
P(J)= 2/10
?

It's the first stage I am really struggling with as he said the total number of cockroaches left was important...

Also would it be possible to calculate the standard deviation for leaving or joining (not together). We have to make a graph and I've done a bar chart (total number joining and leaving in two seperate bars), but I don't see how I can calculate SD to make error bars...

Thankyou!