Questions about combustion and isotopic abundance
$\frac{0.214g}{0.428g} = \frac {x}{0.782g}$
Your answer is ok, but on an exam you want to solve quickly.

2.710g - 1.451g = ?


wasn't that easier?

3.


  35.967(0.00006) + 37.962(x) + 39.962(1-x) = 39.948


This is slightly wrong.

0.00006, x, 0.99994-x