1/64
The half-life of an radioactive substance is the amount of time it takes for half of the substance to decay. So divide 65 years by 10.8 years and you determine that you've spent 6.018518519 half-lives.
Now 2^-6 = 1/64, so you can say that you have 1 sixty-fourth of the original krypton remaining, or you can be more precise and calculate 2^-6.0185 ~ 1/65
