Answer:
21 children
Step-by-step explanation: 6.50x+8.75y=259 
 
This equation came from the cost of the tickets for the children and adults.
x+y=35 
 
This comes from the group of 35 people that consists of children and adults. 
 
We can use substitution to solve for x and y. 
 
x=35-y 
 
6.50(35-y)+8.75y=259 
 
227.5-6.50y+8.75y=259 
 
227.5+2.25y=259 
 
2.25y=31.5
y=14 
 
Now that we have found y, we can plug it into any of the equations to find x. 
 
x+14=35 
 
x=21 
 
Therefore, there are 21 children and 14 adults in the group.