My answers are close to yours but they don't agree. I think that you may have had a sig figs issue. If we look to the part of the equation where you did: 63.546-64.9278=-1.3818This has one sig fig too many because you are only limited to three decimal sig figs (63.546).If you adjust that, you would instead get -1.382: And that would make your answers x=69.16% and 1-x=30.84%. I do have some suggestions. It seems that you just jumped right into the problem. For clarification, next time define your variables. Also, to make things look more professional, use an equation editor.All in all, good job!
Emily, your work looks great but I got different answers then you did. I got 31% and 69% for copper 63 and copper 65, respectively. I agree with what Cyrus has said above. It seems as if you made a sig fig mistake while calculating. Remember when adding and/or subtracting your answers decimal places should go with the one that has the least amount of sig figs. When adding and/or dividing your answers should have the same number of sig figs as the number with the least amount in the original equation. Also you should use sig figs when you are done with a set, i.e only round your sig figs after you have done all the addition in the problem. Good job!