Question

a scientist kept 500 grams of a radioactive material in a container. after 60 minutes, he observed that 62.5 grams of the radioactive material was left in the container. based on the observation, which of these is the most likely inference drawn by the scientist?

Answer 1

After the 1st half-life 250 g of the radioactive material will be left. 
After the 2nd half-life 125 g of the radioactive material will be left. 
After the 3rd half-life 62.5 g of the radioactive material will be left. 

Therefore after 60 minutes, 3 half lives would have occurred. 

Therefore the half-life = 60 / 3 = 20 minutes when 250 grams of the original material would have remained radioactive. 

Therefore the answer is A) About 250 gram of the material decayed in 20 minutes.