My answer to this word problem is: S-2 or it can also be G-2. Do you want to know why?
Explanation: It says on the question that there are a group of students so we don't know how many there are. Therefore we use a Variable to represent an unknown number.
We can use S - Students or G - Group as the variable.
The next thing it says on the word problem is that are 2 friends that left the group! That's why my answer is S-2 or G-2.