Each of these has two unknowns and two equations. One equation says "the total amount of mixture is ...". The other equation says "the proportion in the mixture is..."

Then you use standard methods to solve two equations in two unknowns.

1. Let x = amount of kenyan coffee. The price of x kg of kenyan coffee is 12x.
Let y = amount of turkish coffee. The price of y kg of turkish coffee is 15y.

Together, they make 226 kg. So x + y = 226. There's the first equation.
Together, the mixture sells for $14/kg. So 226 kg sells for 226 * 14. Add up the two individual prices: 12x + 15y = 226*14.

There's your two equations.

2. x = amount of soy meal. Amount of protein in it is 0.18x.
y = amount of corn meal. Amount of protein in it is 0.09y
Total amount of protein in a mixture is 0.18x + 0.09y. This is supposed to be 12% of 360 lb, or 0.12*360.

0.18x + 0.09y = 0.12*360

And x + y = 360. There are your two equations.

3. x = amount of drink 1. Amount of orange in it is 0.15x
y = amount of drink 2. Amount of orange in it is 0.10y.
Total amount of orange in the mixtures is 0.15x + 0.10y. And I hope you can see from the previous problems how the rest of it goes.

let x = klg of kenyan
let y = klg of turkish

x + y = 226
12x + 15y = 14(x + y)

let x = lbs of soy
let y = lbs of corn

.18x + .09y = .12(x + y)
x + y = 360

let x = L of 15% juice
let y = L of 10% juice

.15x + .1y = .12(x + y)
x + y = 5