Here we are given that the amount if radioactive particles has come down to 40,000 from 80,000 in 80 years . Hence by the definition of Half Life,the half life of the item is 80 years.
Now we are asked to guess the number of radioactive particles after 80 years. As we already know that , the half life of material is 80 years , the number of radioactive particles will be reduced to its half in 80 years. Hence the number of radioactive particles after 80 years is 20,000
