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 = (780 g)(e^-0.163*16)
A = 57.5 g
