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**A radioactive nucleus has a half-life of 20 years, starting with 100,000 particles, how many particles will be left exactly at the end of 40 years?**

- A.
**75,000 particles** - B.
**35,000 particles** - C.
**25,000particles** - D.
**50,000particles**

**QUICK ANSWER…**

**C**

**DETAILS… **

**If the half-life = 20 years, it means at the end of every 20 years, half of the particles at the beginning would have decayed.**

**Therefore, at the end of 20 years, 100,000 particles would decay into 50,000 particles,**

**At the end of another 20 years, 50,000 particles would decay into 25,000 particles.**

**So, at the end of a total of 40 years, 75,000 particles would decay, and 25,000 would remain.**

**Now for the right answer to the above question:**

**Option A is incorrect. 75,000 particles decayed, but 25,000 remained****Option B is incorrect.****C is correct.****D is not the correct answer. This should be the remnant after 20 years and not after 40 years.**

**KEY-POINTS…**

**You may please note the****se/this****:**

**Please note the difference between the remnant and the decayed, in the above question, at the end of 40 years, 75,000 particles decayed, and 25,000 particles remained.****Similarly, after a total of 60 years, 12,500 particles would be remaining and 87,500 particles would have decayed.**

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**/ culled from 2019 JAMB-UTME Chemistry question 35 /**