Let's assume that Mr. and Mrs. Johnson bought x adult tickets and y children's tickets.
Since there are three children, we know that:
y = 3x
The total cost of the parents' tickets is 7x, and the total cost of the children's tickets is 4y. Substituting y = 3x, we get:
total cost of parents' tickets = 7x
total cost of children's tickets = 4(3x) = 12x
Therefore, the ratio of the total cost of the parents' tickets to the total cost of the children's tickets is:
(7x) / (12x) = 7/12
So the answer is 7/12, which cannot be simplified further. Therefore, the ratio is 7/12, which can be written in the form aib as 0+7/12i.