The answer? S-2 or G-2
well the question says "a group of students" it doesn't tell us how many students are in the group. Therefore we use a Variable since the number is unknown.
Also it is -2 because the question states that 2 students Leave the group.
S stands for Students &
G stands for Group.
You could use either one in this question.
To know what a Variable is Click here to View my blog post on what a Variable is.