Aproblem states: "there are 9 more children than parents in a room. there are 25 people in the room in all. how many children are there in the room? "

let c represent the number of children.

which expression represents the number of parents?

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)

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

Number of children = 17
Number of parents  = 8

the law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. in other words, a given compound will always contain the same elements in the same proportions by mass.the law of multiple proportions states when two elements combine with each other to form more than one compound, then the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.