Question 703263
Radioactive decay may work like an exponential decay.
Using t for years, A[0] for starting amount, A[t] for amount at time t,
We may use formula, {{{A[t]=A[0]e^(k*t)}}}


It's not clear if you mean first 4 years and then 3 more years, or are you just saying, "half life is 4 years"+".  After 7 years."


Starting with half life, we want to find k.
In a 4 year period, t=4, and A quantity will be reduced to (1/2)*A.
{{{(1/2)=1*e^(k*4)}}}
{{{ln(1/2)=ln(e^(k*4))}}}
{{{ln(1/2)=k*4}}}
{{{k=(ln(1/2))/4}}}


Computing k gives us k = 0.173.
Our decay formula then is {{{A[t]=A[0]*e^(-0.173*t)}}}



NOW, if you want the quantity remaining after 7 years and the starting quantity is 12 grams, then:
{{{A[7]=12*e^(-0.173*7)}}}
{{{A[7]=12*0.2979}}}
A[7]=3.6 grams