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This problem is a SPECIAL CASE: one equation is combined with one inequality.
The equation is
x + y = 120. (x for adults, y for children)
The inequality is
500x + 400y <= 80000
Simplify this inequality by dividing both sides by 100
5x + 4y <= 800.
So, you have, actually, this system (one equation and one inequality)
x + y = 120
5x + 4y <= 800
Also, the problem assumes that both quantities x and y are NON-NEGATIVE
x >= 0, y >= 0.
On the plot, it looks like this
Plot y = 120-x (red line) and y = (green line)
The inequality represents all the points inside the triangle in QI under the green line.
The equation represents the red line.
So, the range for x is 0 <= x <= 120.
The range for y is the same 0 <= y <= 120.
But x and y are not independent variables: they are interconnected by the equation x + y = 120.
This plot allows you to see the problem in whole and its solution, in particular.
The solution set is { the INTEGER points of the red line in QI }.
It is the full analysis, presented in the form AS IT SHOULD BE DONE.