We will use the next variables
x is the number of tickets sold for students
y is the number of tickets sold for adults
First equation
x+y=150
Second equation
3x+8y=1000
we isolate the x of the first equation
x=150-y
we substitute the equation above in the second equation
3(150-y)+8y=1000
450-3y+8y=1000
we sum like terms
5y=1000-450
5y=550
y=550/5
y=110
Then we substitute the value of y in the equation with x isolate
x=150-y=150-110=40
x=40 and y=110
The students sold
40 tickets for students
110 tickets for adults