Answer:
The system of equations that could be used to find the cost of one adult ticket and one child ticket is:
 

On solving we get:
Cost of one adult ticket= $ 10
and cost of one child ticket= $ 3
Explanation:
a denote the cost of one adult ticket.
and c denote the cost of one child ticket.
 Roy buys 6 adult tickets and 2 child tickets for a total of $66. 
i.e. the equation that could be written is:
 6a+2c=66-----------(1)
and also Elisa buys 5 adult tickets and 4 child tickets for a total of $62. 
This means that the equation that is obtained from this information is:
 5a+4c=62--------------(2)
Also, on solving the two equations by the method of elimination
we multiplying equation (1) by 2 and subtract both the equations we get:
 12a+4c=132
and 5a+4c=62
-----------------------------------
 7a=70
i.e. a=10
and on putting the value of a in equation (1) we get:
 60+2c=66
i.e. 2c=66-60
i.e. 2c=6
i.e. c=3
Hence, the cost of one adult ticket=$ 10
and cost of one child ticket= $ 3