JAMB Physics 2019 Objective — Question 33

Question 33 of 40 from the Joint Admissions and Matriculation Board (JAMB) Physics 2019 Objective paper, with the correct answer and a full explanation.

A certain radioactive substance has a half life of 20 years. How long will it take the activity to become 1/4 of its original?

A. 20years
B. 40yearsCorrect
C. 60years
D. 80years

Explanation

Decay constant λ=0.693/t½=0.693/20=0.034651/yr. When activity is ¼ of original: ¼=e^(−λt) ⇒ −λt=ln(¼)=−1.3863 ⇒ t=1.3863/0.034651≈40 years.

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