# Help solving a system of linear equations

I haven't done this in a long time so please bare with me.

I need to solve this system to find the percent abundance of 2 isotopes.

68.9257x + 70.9249y=69.723
x+y=1

In order to solve the system by addition/subraction one would multiply 68.9257 through the second equation, then procede to subract right? I've been doing this, but I haven't been getting the right answer........

Help............ ...which should work. Then substitute the value of y you get into either equation. This method's called Gaussian Elimination followed by back substitution.

Hurkyl
Staff Emeritus
Gold Member
Well, if you show your work, maybe we can find your error.

HallsofIvy
Homework Helper
"I haven't done this in a long time so please bare with me."

Hey, this isn't that kind of website!

arildno
Homework Helper
Gold Member
Dearly Missed
The simplest way in this case is:
1. Rewrite your second equation as y=1-x
2. Substitute this expression for "y" into "y"'s place in the first equation
3. Collect the "x"'s on the left-hand side on your equation, and the "constants" at your right hand side.
4. Divide the resulting equation on both sides with the numerical factor appearing in front of your "x" symbol.
The resulting equation will look like:
x=some number.
5. Put "some number" into "x"'s place in the equation y=1-x to find the y-value

Thank you very much guys, I seem to have found my mistake.