On a trip, the fare was 50¢ for each adult and 25¢ for each child. If 30 passengers paid $12.25, how many adults and children went?

Which of the following equations could not be used to solve the problem?

50(30 - x) + 25 x = 1225
25 x + 50 x = 30(1225)
50 x + 25(30 - x) = 1225