Let X be the cost of an adult ticket.
So the cost of a child ticket is X-6.
The total cost of the 5 tickets is X*2 + 3*(X-6) = 77.
X*2 + 3*X - 3*6 = 77.
5*X - 18 = 77.
5*X = 77 + 18 = 95
X = <<19=19>>19
#### 19