The admission fee for a charity event is $7 for children and $10 for adults. The event was attended by 700 people, and the total amount collected in admissions was $6,400.There were _children and _adults at the show.

If all were children, revenue would be 700*$7 = $4900. Revenue is actually $6400 -4900 = $1500 more than that. Each adult admission that replaces a child's admission adds $10 -7 = $3 to the revenue, so there must have been
  $1500/$3 = 500 . . . . adult admissions

There were 200 children at the show.
There were 500 adults at ths show.