The constraints are:
x+y≤30
x+y≥15
y≤20
x≤15
x≥0 and y≥0
The corner points occur where two boundary lines intersect. The boundaries are:
x+y=30, x+y=15, y=20, x=15, x=0, y=0
Where x=0 meets x+y=15:
0+y=15
y=15
Point: (0,15)
Where x=0 meets y=20:
Point: (0,20)
Where y=0 meets x+y=15:
x+0=15
x=15
Point: (15,0)
Where x=15 meets x+y=30:
15+y=30
y=15
Point: (15,15)
Where y=20 meets x+y=30:
x+20=30
x=10
Point: (10,20)
The corner points of the feasible region are: (15,0), (15,15), (10,20), (0,20), (0,15)