Answer: The concentration of cow's milk after 5 years is 10691 Bq/L
Step-by-step explanation:
All the radioactive reactions follow first order kinetics.
The equation used to calculate rate constant from given half life for first order kinetics:

We are given:

Putting values in above equation, we get:

The equation used to calculate time period follows:

where,
= initial concentration of Cow's milk = 12000 Bq/L
N = Concentration of cow's milk after 5 years = ?
t = time = 5 years
k = rate constant =

Putting values in above equation, we get:

Hence, the concentration of cow's milk after 5 years is 10691 Bq/L