Here is the answer to the given problem above.
Let x be the number of parents and y for the children.
x + 3y = 40
2x + 3y = 53
x = 40 - 3y
2(40-3y) + 3y = 53
80 -6y + 3y = 53
80 - 3y = 53
-3y = 53 - 80
-3y = -27 (divide both sides by -3)
y = 9
This means that the cost per child is $9
Now solve for x
x = 40 - 3y
x = 40 - 3 (9)
x = 40 - 27
x = 13
So the cost per parent is $13.
Hope this answers your question.