We have a problem of a sysmtem of equation
x is the number tickets sold of general admission
y is the number o tickets sold for seniors
The first equation is about the number of ticktes sold
x+y=155
the second equation is about the amount of money
12x+9y=1680
we isolate x of the first equation
x=155-y
we substitute the equatio above in the second equation
12(155-y)+9y=1680
1860-12y+9y=1680
we isolate the y
-3y=1680-1860
-3y=-180
y=-180/-3
y=60
then we substitute the value of y in order to find x
x=155-y
x=155-60
x=95
They sold 95 tickets of general admission