3 bags of Brand X dog food and 4 bags of Brand Y dog food costs $18, while 4 bags of Brand X dog food and 3 bags of Brand Y dog food costs $17. What is the cost of each bag of Brand X?
3x + 4y = 18
4x + 3y = 17 Multiply this equation by - 4/3 (we are trying to eliminate 'y' when we add the equations)
-16/3 x - 4y = - 68/3 add this equation to the top one
-7/3 x = -14/3
x = $ 2 / bag