I had this exam in January this year and there was a problem I didn't know how to solve, but I know these type of problems are pretty easy so I thought maybe someone here can try and explain it to me.
In a group of students, 50 are from Sweden, 15 from India, 15 from Pakistan and 10 from Iran.
Four students are selected at random (without replacement).
Calculate the probability that one from each country is selected.
I this case you divide the number of ways to pick one each country by the total number of ways to pick four period.
If you do not understand how to do that you must realize that are we are not tutorial service.
So you must rely on your notes.