Random problem

"The Dutch Nook sells two brands of coffee: one for $12.90 per kg, the other for $4.40 per kg. The store owner mixes the 2 brands of coffee and sells the mixture for $12.00 (in total). It is given that the mixture weighs 2kg. How many kg of each brand should she use in order to make the same revenues if the two brands were sold unmixed."

You got two things that you don't know:  mass of brand A and mass of brand B.  Whenever you have two things that you don't know, you need two equations that relate the two things.

One equation can come from the money involved:
12.90A + 4.40B = 12

One equation can come from the masses:
A + B = 2

Do you know how to proceed from that?