Question 706112
Every time period, the material lost 1.1% of what it had in the beginning of the period.  Meaning that it becomes 100-1.1=98.9 percent of its strength as each time period passes.  


Starting at any amount A, the first period makes A*0.989, the second time period means that there is {{{A*0.989*0.989}}}, and then for any number of time periods, the quantity remaining will be {{{A*(0.989)^t}}}, where t is the number of time periods which pass.


If {{{A[0]}}} is the initial amount to start, the {{{A[t]}}} is the amount remaining after t time.  t can be in years, in this example.
{{{A[t]=A[0]*(0.989)^t}}}


If you want what t for starting with 250 grams and having 100 grams remaining,
{{{100=250*0.989^t}}}
{{{2/5=0.989^t}}}
{{{ln(2/5)=t*ln(0.989)}}}
{{{t=(ln(2/5))/(ln(0.989))}}}


Seems to be t=82.8 years  or t=83 years.