Chemistry
posted by Josh on .
x+y=9.0
0.50x+0.20y=3.90
What is the value of x and y?

I would do this.
x+y = 9.0
0.50x + 0.20y = 3.90
Multiply eqn 1 by (1/2).
0.50x  0.50y = 4.50. keep eqn 2 as is.
0.50x + 0.20y = 3.90
Now add the two. You see the x term cancels. Solve for y, then substitute y back into one of the equation and solve for x.