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Let a = # of packs of Coffee A;
b = # of packs of Coffee B;
c = # of packs of Coffee C.
Then, counting the balances for each type of coffee beans, you have this system of 3 equations in 3 unknowns a, b and c:
0.3a + 0.2b + 0.1c = 30 (kilograms of Arabica) (1) (<<<---=== counting Arabica)
0.2a + 0.1b + 0.2c = 25 (kilogram of Robusta) (2) (<<<---=== counting Robusta)
0.2b + 0.2c = 15 (kilogram of Kenya) (3) (<<<---=== counting Kenya)
There are many ways to solve it.
I chose the simplest way, using the online solver of this site
https://www.algebra.com/algebra/homework/Matrices-and-determiminant/cramers-rule-3x3.solver
giving me the answer a= 65, b= 30 and c= 45.
The last step that I did was checking:
0.3*65 + 0.2*30 + 0.1*45 = 30 ! Correct !
0.2*65 + 0.1*30 + 0.2*45 = 25 ! Correct !
0.2*30 + 0.2*45 = 15 ! Correct !
Solved.