Answer: Atoms of
left in
a) 
b) 
c) 
Explanation: The given reaction is a type of radioactive decay and all the radioactive decay follows first order reactions. Hence, to calculate the rate constant, we use the formula:



To calculate how much amount of sample is left, we use the rate law expression for first order kinetics, which is:
....(1)
where,
k = rate constant = 
t = time taken for decay process
= initial amount of the reactant = 
N = amount left after decay process
t = 5 years
Putting values in equation 1, we get:

t = 30 years
Putting values in equation 1, we get:

t = 190 years
Putting values in equation 1, we get:
