To create a one-variable, one-step equation based on the information provided, we can let \( a \) represent the number of adult tickets sold. The problem states that the theater sold 15 more adult tickets than child tickets, and the total number of child tickets sold was 42.
The equation can be set up as follows:
\[ a = 42 + 15 \]
However, the form you provided needs to represent the relationship as follows:
So the correct setup is:
\[ a - 15 = 42 \]
Now, to fill in your blank in your equation format:
The equation should be: \[ a [=] 15 + 42 \]
So the correct symbol to fill in is \( = \).
This means the complete answer is:
\[ a [=] 15 + 42 \]