As per the problem 
There are 146 students 5 teachers, and 8 chaperones going to the theatre.
Each row has 8 seats.
So Total number of seats required

Now to know the number of rows required, divide the total number of seats required by capacity of one row
Number of Rows required to reserve 
 
 
Since as per the problem, To reserve their seats, they need to reserve entire row.
So Number of Rows to reserve=20