Question

What amount of a radioactive substance will reduce to 5 g in four half lives?

Answer 1

1 st four months - 1/2 gone

2nd four months = 1/2 or 1/2 gone = 1/4 gone ; total gone =1/2 + 1/4 = 3/4 gone (decay)

So, 8 months for 3/4 gone (2 half life periods)