So what you are trying to find is the certain amount of children tickets and adult tickets. the final amount is a total of $12. This means that a unknown number of children tickets (c) times 1.50 (price of each ticket for children) plus the unknown number of adult tickets (a) times 4.00 (price of each ticket for adults) equals a final cost of $12.00.
Equation: 1.50c+4.00a= 12.00