Given :
An alloy is a mixture of metals. Suppose that a certain alloy is made by mixing 50 g of an alloy containing 12% copper with 78 g of an alloy containing 92% copper.
To Find :
How many grams of copper are in the resulting mixture what percentage of the resulting mixture is copper.
Solution :
Mass of copper in 50 gm alloy =
.
Mass of copper in 78 gm alloy =
.
Total mass of copper, ( 6 + 76.44 ) gm = 82.44 gram.
Percentage of copper in resulting mixture :

Hence, this is the required solution.
Use elimination and subtitution method to solve the problem.
First, eliminate x and you'll find the value of y
x - 4y = 12
x - y = 0
--------------- - (substract)
-3y = 12
y = 12/-3
y = -4
Second, subtitute -4 as y and you'll find the value of x
x - y = 0
x- (-4) = 0
x + 4 = 0
x = -4
The solution
x,y = -4,-4