A radioactive Cobalt 60 has a half life of 5.25 years.
Therefore; the fraction that will remain after 26.35 years will be;
26.35 years
New mass = original mass × (1/2)^n
Where n is the number of half lives
n = 26.35/5.25
= 5.019
If the original mass is 1 whole, then the new mass would be;
= 1 × (1/2)^5.019
= 0.03084
≈ 0.03 or 3/100
Therefore, an approximate fraction of 3/100 will remain