A store mixed
Starbucks w/ $11/kg & Dunkin Donut worth $13/kg.
The mixture is to sell $12/kg.
Find how much of each should be used to make 420 kg mixture.

Let x= Starbucks
Let y= Dunkin Donuts

x+y=420
11x+13y=(12)(420)
11x+13y= 480

I am stuck at this point....

Question

A store mixed 
Starbucks w/ $11/kg & Dunkin Donut worth $13/kg.  
The mixture is to sell $12/kg.
Find how much of each should be used to make 420 kg mixture.

Let x= Starbucks
Let y= Dunkin Donuts

x+y=420
11x+13y=(12)(420)
11x+13y= 480

I am stuck at this point....

anonymous · Answer

Correction:  The last line should be 11x+13y=5040

rulnick · Answer

210 kg each.  They must be mixed in equal parts if the cost is to be half-way between the two costs.

rulnick · Answer

By substitution: From the first equation, y=420-x. Substitute 420-x for y in the second equation and solve for x to get x=210.

rulnick · Answer

Or, by linear combination, subtract 11 times the first equation from the second to get 2y=420, or y=210.