El máximo común divisor es 12
Por cierto, hablo inglés, así que estoy usando Google Translat. :)
Number of Adults = a
Number of children = 2a
Number of people in a group = 45
so
a + 2a = 45
3a = 45
a = 15
Number of Adults = 15
Number of children = 30
1. Simplify the inequality 

2. Separate terms with x and without x in different sides:

3. Add similar terms:

4. Divide by 21.42:

Answer: correct choice is C.
The answer is $1,051.17 if I’m not mistaken
I think it is a I believe but I could be wrong so correct me if I’m wrong