Answer: the cost of a children’s ticket is $5
Explanation:
Let x represent the cost of one child ticket.
Let y represent the cost of one adult ticket.
An adult ticket to a museum cost $3 more than the children's ticket. This means that
y = x + 3
when 200 adult tickets and 100 children’s tickets are sold, the total revenue is $2100. It means that
100x + 200y = 2100- - - - - - - - -1
Substituting y = x + 3 into equation 1, it becomes
100x + 200(x + 3) = 2100
100x + 200x + 600 = 2100
300x = 2100 - 600
300x = 1500
x = 1500/300
x = 5