Based on the given information, Martin asked his 7 co-students. Together with him, they will be 8.
Let's multiply 8 by the average personal expenses which is $2, 850.
Hence, for them to have an average of $2,850 in their personal expenses, their total expenses must be $22,800.
Now, let's add the expenses of the 7 students.
The total expense of the 7 students is $19, 750.
Let's subtract $19, 750 from $22, 800.
Therefore, Martin's actual personal expenses for that school year have to be $3, 050 so that his amount and his co-students amounts together would have an average of $2,850.