What is the Natural abundance of Potassium The three naturally occurring isotopes of potassium are $\ce{^39K}$ ($38.963707\:\mathrm{amu}$); $\ce{^40K}$ ($39.963999\:\mathrm{amu}$); and $\ce{^41K}$.
  
  The percent natural abundances of $\ce{^39K}$ and $\ce{^41K}$ are $93.2581\%$ and $6.7302\%$ respectively.
  
  a) What is the natural abundance of $\ce{^40K}$? 
  
  b) Determine the isotopic mass of $\ce{^41K}$. 


So I started to utilize this equation:

$\mathrm{atomic\:mass = (fractional\:abundance\:isotope\:1 
\cdot mass) + (fractional\:abundance\:isotope\:2 \cdot mass)}$

But then I realized that I did not have some values. $\ce{^39K}$ is the only isotope which has two values. $\ce{^40K}$ only has the mass weight, however, if I were to multiply $\ce{^40K}$'s mass with ($1-0.932581$) would that help me arrive at an answer? 

Also, would that answer be accurate since it only takes into account that there are only to values that add up to 100%.
A). If you know the percentages of the abundance of $\frac{2}{3}$s of the isotopes then finding the percentage of the $3^{rd}$ is as simple as 

$ 100\% - (\%_{\ce{^39K}}+\%_{\ce{^41K}}) = \%_{\ce{^40K}} $

B). Finding the mass of the isotope should be pretty simple. It would be

$(\mathrm{Mass\ of\ proton} \cdot \mathrm{{\#\ of protons}} )+(\mathrm{Mass\ of\ a\ neutron} \cdot \mathrm{{\#\ of neutrons}} ) \approx \mathrm{Mass\ of\ isotope}$

You neglect the weight of electrons because the mass is small relative to the others.