Write the system of equation based on the problem
"A total of 345 tickets that consists of adult tickets and students ticket were sold" could be written as:
∴ a + s = 345 (first equation)
"the number of student tickets sold was two times the number of adult tickets sold" could be written as:
∴ s = 2a (second equation)
Solve the system of equation
Substitute 2a as s in the first equation
a + s = 345
a + (2a) = 345
3a = 345
a = 345/3
a = 115
There are 115 adult tickets sold.
