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Earlier discussion
2 older comments kept for context.
Favour Onwuchekwa
\( Half life (t_{½})= 20years
This implies that half (½) of the molecules of the substance decay every 20 years.
\)\( After the first 20 years, half of the molecules of the substance would have decayed;
\frac{1}{2}×100,000=50,000
The remaining particles would be 100,000–50,000=50,000 particles.
In the next 20 years Making it a total of 40 years, half of the molecules would have also decayed;
\frac{1}{2}×50,000=25,000 particles
\therefore After 40 years the particles remaining would be 25,000 particles. [C] \)
Doctor Strange
Given:
t\(_{\frac{1}{2}\) = 20 years
After the first 20 years, half of the substance (\(\frac{1}{2} \times 100,000 = 50,000\)) 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 (\(\frac{1}{2} \times 50,000 = 25,000\)) will have decayed.
Remaining particles after 40 years = 50,000 - 25,000
= 25,000 particles.