Question

The sum of two numbers is 12. The first number,x, is twice the second number ,y,

Answer 1

Judging by the question at hand I generated this equation.
x+y=12
x=2y

I begin this question by plugging in the x=2y into the equation for x. 
So the new equation should be 3y=12. I then divide the entire equation by 3 to get y=4.

Next I plug y=4 into the equation, the new equation should be x+4=12. I then subtract 4 from both sides to get x=8.

The two numbers are :
x=8 y=4