Question 1010625
You have only one datum point in the half-life information, and then a value for making a question.  Why not the basic exponential decay equation like {{{y=pe^(-kt)}}}?


Try your simpler format anyway.
Half-life information would seem to make  {{{(1/2)=1*b^50}}}, using a for the initial quantity of Cs-142.


{{{b^50=1/2}}}


{{{log(10,(b^50))=log(10,(1/2))}}}


{{{50*log(10,b)=log(10,(1/2))}}}


{{{log(10,b)=(1/50)log(10,(1/2))}}}


{{{log(10,b)=-0.0060206}}}


....
{{{highlight(b=0.986)}}}
The model being  {{{highlight(y=a(0.986)^x)}}}, using a as the initial quantity.
This b value is not a growth factor!!  The "growth" is negative, not positive.


Having "twenty grams now", you can write {{{y=20(0.985)^x}}}.