the problem is $\displaystyle .055=(.0100-2x)^2/x^2$. This becomes $\displaystyle .055=(.0001-4x^2)/x^2$. I multiplied both sides by$\displaystyle x^2$ to get $\displaystyle .055x^2=.0001-4x^2$, then added $\displaystyle 4x^2$ to each side to get $\displaystyle 4.055x^2=.0001$, divided both sides by 4.055 to get $\displaystyle x^2=2.466*10^-5$ and took the square root of both sides to get x=.004965 or $\displaystyle 5.0*10^-3$. My book however says the answer is $\displaystyle 4.4*10^-3$ so what have I done wrong? This is actually a chemistry problem so 4.9 to 4.4 is a bigger difference than I am comfortable with.