Let c = number of children in the room. Let p = the number of parents in the room.
There are 9 more children than parents in the room. Therefore c = p +9 (1)
There are 25 people in the room. Therefore c + p = 25 (2)
Answer: The expression for the number of parents is p = 25 -c
------------------- Note: We can determine the number of children and parents. Substitute (1) into (2). p + 9 + p = 25 2p + 9 = 25 2p = 25 - 9 = 16 p = 16/2 = 8 c = p + 9 = 8 + 9 = 17