Answer:
A = 66 adults, C = 40 children
Explanation:
We can set up a system of equations to find how many adults and how many children attended the concert, with A representing the adults and C representing the children:

We can use the elimination method and multiply everything in the first equation by -4

Once we add the values together, we're left with:
, where solving for C gives us C = 40
We can plug C into the first equation to find A:
, where solving for A gives us A = 66