for a half- life, the formula is
A(t) = A0(0.5)^( t / t_0.5)
where A(t) is the amount of substance at time t
A0 is the initial amount of subtance
t_0.5 is the time it will reduce its amount to half
t is the time
substitute the given to the formula
A(t) = 100(0.5)^( t/6 )
so the decay factor is 0.5^( t/6 )