6 adults and 11 children.
In order to solve this problem we going to use linear equations.
A group of 17 people went to the concert. There are adults and children in that group x + y = 17 where x are adults and y are children. That group pay $57.50 for tickets, if tickets cost $5.00 for adults and $2.50 for children, then 5.00x + 2.50y = 57.50.
x + y = 17 ----------> y = 17 - x
Substituting the value of y in the equation 5.00x + 2.50y = 57.50:
5.00x + 2.50(17 - x) = 57.50 solving
x = 6
Substituting x = 6 in the equation x + y = 17
6 + y = 17 solving
y = 11
From a group of 17 people who went to the concert 6 are adults and 11 are children.