Given:
t12 = 20 years
After the first 20 years, half of the substance (12×100,000=50,000) will have decayed. Hence, we have 100,000 - 50,000 = 50,000 particles left.
After the second 20 years (being 40 years in all), half of the remaining substance (12×50,000=25,000) will have decayed.
Remaining particles after 40 years = 50,000 - 25,000
= 25,000 particles.
