Answer: 30 marbles
Reason
S = sum of the marbles of all 6 kids
S/6 = 25 since we divide the sum over the number of kids to find the average.
S = 25*6
S = 150
All 6 kids have a total of 150 marbles.
4 of them have an average of 30 marbles, so those four students must have a subtotal of 4*30 = 120 marbles based on similar logic as shown above.
The other two kids have 150-120 = 30 marbles collectively between them.
Side note: I'm assuming you meant to ask "how many marbles do the remaining 2 kids have altogether?" instead of "how many students do the remaining 2 kids have altogether?"