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1-st constraint x + y <= 8.
Consider equation x + y = 8 and plot the line y = 8-x.
Do it ON YOUR OWN.
2-nd constraint 5x + y <= 20.
Consider equation 5x + y = 20 and plot the line y = 20 - 5x.
DO IT ON YOUR OWN.
3-rd constraint x >= 0.
It is LEFT half-plane (everything to the left of y-axis).
4-th constraint y >= 0.
It is UPPER half-plane (everything above the x-axis).
The 3-rd and the 4-th constraint together define (is) Quadrant I.
P = 6x + 4y is the profit function (or "objective" function).
We do not plot it.
Now your domain is the quadrilateral in the first quadrant under
the line y = 8 - x and the line y = 20-5x.
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I answered all your questions.
Make the plot and REPORT me if everything is clear to you . . .
If you will respond, refer to the problem ID number 1147061.
Otherwise I will not know to which problem relates your response.
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To see many solved problems on Linear Programming method, look into my lesson
- Solving minimax problems by the Linear Programming method
in this site.