The half life of cobalt-60 is 5.27 years, approximately how much of a 199 g sample will remain after 20 years

Radioactive decay is expressed by the following formula:

N₀ is the initial number of undecayed atoms.
Nt is the number of undecayed atoms remaining after time t
λ is the decay constant.
The relationship between λ and the half life time t1/2 is:
λ =  = 0.693 / 5.27 = 0.1315 
Taking natural logs for both sides of the decay expression:
ln Nt = ln N₀ - λ t
ln Nt = ln 199 - (0.1315 x 20)
        = 5.293 - 2.63 = 2.66
From which:
Nt = 14.29 g

Both molecules are alkanes. But, methane is small molecule with molecular mass M = 16 g/mol (Ar(C)=12, Ar(H)=1) and octane is larger molecule with molecular mass, M = 114 g/mol (8·12 + 12·1). The smaller and lighter is the molecule, movement and space between molecules increased, so methane is a gas.