Answer: $5
Explanation:
Let the cost of one adult ticket be x and the cost of one child ticket be y , then from the first statement:

And from the second statement , we have

combining the two equations , we have 
 ........................... equation 1
 ........................... equation 1
 ........................... equation 2
 ........................... equation 2
Solving the system of linear equation by substitution method , from equation 1 , make y the subject of the formula ,that is
 ............................... equation 3
 ............................... equation 3
substitute equation 3 into equation 2 , equation 2 then becomes
 =
 = 





Substitute 
 into equation 3 to find the value of y
 into equation 3 to find the value of y




Therefore : the cost of one child ticket is $5