Question

a store mixes brazilian coffee worth $12 per kilogram and turkish coffee worth $15 per kilogram. the mixture is to sell for $14 per kilogram. find how much of each should be used to make a 312-kilogram mixture

Answer 1

Think of this as number of items vs value of the items. You need to make an equation for each situation.
Let B be the number of kilograms of Brazilian coffee and T the number of kilograms of Turkish coffee.
Then:
1) B +T = 312 ( this is the total number of kilogram of the mixture)
2) 12B +15T= 312x14 (this is assuming that they priced the mixture proportionally to the value of its components)
Here you go! you have your two equations and two unknown. Use elimination or substitution to get B and T and that the answer you are seeking.