Exponential decay is a very common process especially when we are talking about radioactive materials. So, there is already a common formula for this type of behavior which is written below:
A = Pe^-rt
where
A is the amount left after time t
P is the initial amount at t=0
r is the rate
Substituting the values,
A = (9575 kg)(e^-0.12*5)
A = 5,254.87 kg
