Question 1139313
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How many cases of each brand should the breeder mix together to obtain a food that provides 3950 units of​ fiber, 3060 units of​ protein, and 2740 units of​ fat? 
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Let x represent the number of cases of Brand​ A, y represent the number of cases of Brand​ B, z represent the number of cases of Brand​ C, and w represent be the number of cases of Brand D. There are four ways in which the breeder can mix brands to obtain a food that provides 3950 units of​ fiber, 3060 units of​ protein, and 2740 units of fat.
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<pre>
BRAND       Variable       Fiber        Protein         Fat
A              x            25            30            30
B              y           100            80            70
C              z           275           210           190
D              w           100            80            60
Needed                    3950          3060          2740
</pre>

{{{system(25x+100y+275z+100w=3950,30x+80y+210z+80w=3060,30x+70y+190z+60w=2740)}}}


This is not a finished solution; but just a system of equations according to the described information and the given question.
This has one fewer equation than number of variables.