The first thing you need to do is determine the abundance of each of the two isotopes. Since you're only dealing with two isotopes, you can express the abundance of one as x and the abundance of the other as 1 - x.
This means that you'll get
Ne-20 will have a percent abundance of 90.5% and Ne-22 will have a percent abundance of 9.5%.
SIDE NOTE You get percent abundance by multiplying the decimal abundance by 100.
So, for 1 mole of neon, you get 0.905 moles of Ne-20 and 0.095 moles of Ne-22.
Determine how many moles of neon you have in 12.55 g by using its molar mass
This means that you get
You know that 1 mole of any substance contains exactly
Rounded to four sig figs, the answer will be