The admission fee of a theater is 2.50 for adults and 1.25 for children. on a certain day, 700 people went to the theater for a concert and 1375 was collected. how many children and how many adults attended the concert

 let children be Y and adults be X 
we are told 700 people went in total hence 
x+y=700
 adults paid $2.50 and children paid $1.25 and in total we collected $1375 hence;
2.50 x + 1.25 y = 1375 
generating two equations
x + y = 700 
2.5 x + 1.25 y = 1375