The answer is 21x-25 because all you have to do is multiply the 1/4 by the 84x and then multiply 1//4 by negative 100. A simpler way of explaining this is just that you need to distribute
To solve this, I would just find what two adult tickets and one child ticket wold be worth.
To find that, just do:
5.20 + 9.90 * 2, which makes 25.
So now you just find how many 25s are in 900, or 36.
So 36 children's tickets were sold that day.
Hope I helped :)
