Unit Breeder

How do I finish this question?

An animal breeder can buy four types of tiger food. Eac case of brand A contains 25 units of fiber, 30 units of protein, and 30 units of fat. Each case of brand B contains 50 units of fiber, 30 units of protein and 20 units of fat. Each case of brand C contains 75 units of fiber, 30 units of protein and 20 units of fat. Each case of brand D cotains 100 units of fiber, 60 units of protein and 30 units of fat. how many cases of each brand should te breeder mix together to obtain a food that provides 1200 units of fiber, 600 units of protein and 400 units of fat?
q+2r+3s+4v=48
q+r+s+2v=20
3q+2r+2s+3v=40
I’ve gotten this far in answering the exponential equation question but I have no idea how to proceed.

A 25 – fiber, 30 -protein, 30 – fat
B 50 fiber, 30 – protein, 20 – fat
C 75 fiber, 30 -protein ,20 -fat
D 100 fiber, 60 – protein ,30- fat.
question= 1200 fiber, 600 protein 400 units fats

let Q contains aA+bB+cC+dD

25a+50b+75c+100d=1200
30a+30b+30c+60d=600
30a+20b+20c+30d=400

here we have to find 4 variables i.e. a,b,c,d but we have only 3 equations so it is impossible
you must have n number of different equations to find n variables !!

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