Given:
$7/child
$10/adult
Total people = 700
Total money = $6,400
First, make two equations.
Let a = # of adults & Let c = # of children.
Let p = total people
1. a+c = 700
2. 10a+7c = 6,400
Then, rearrange the equation to solve for a variable.
c = 700-a
Substitute (700-a) for c, or the # of children in the second equation.
10a+(700-a) = 6400
9a+700 = 6400
9a+700-700 = 6400-700
9a = 5700
9a/9 = 5700/9
a = 633
= # of adults attended
700-633 = c = 66 
= # of children attended
$7/child
$10/adult
Total people = 700
Total money = $6,400
First, make two equations.
Let a = # of adults & Let c = # of children.
Let p = total people
1. a+c = 700
2. 10a+7c = 6,400
Then, rearrange the equation to solve for a variable.
c = 700-a
Substitute (700-a) for c, or the # of children in the second equation.
10a+(700-a) = 6400
9a+700 = 6400
9a+700-700 = 6400-700
9a = 5700
9a/9 = 5700/9
a = 633
700-633