We have the next variables
x--- people in the lower deck
y-- people in the upper deck
the first equation is
x+y=400
this equation is the number of people that go to the game
The second equation is
x(15)+y(10)=4650
15x+10y=4650
this equation represents the total sales
In order to solve the system of equation, we will isolate the x of the first equation
x=400-y
then this equation will substitute in the second equation
15(400-y)+10y=4650
6000-15y+10y=4650
then we sum similar terms
5y=4650-6000
5y=1350
y=1350/5
y=270
then we substitute the value of y in the equation with the x isolate
x=400-y
x=400-270
x=130
the solution is
130 lower deck seats were sold
270 upper deck seats were sold