JEE Main 2016 · 3 Apr · Shift 1 · Physics Q22

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Half-lives of two radioactive elements A and B are 20 minutes and 40 minutes, respectively. Initially, the samples have equal number of nuclei. After 80 minutes, the ratio of decayed numbers of A and B nuclei will be :

1 : 16

4 : 1

1 : 4

5 : 4

Answer and worked solution

Official answer: D

In 80 minutes A goes through 4 half-lives (decayed $\frac{15}{16}N$ ) and B through 2 ( $\frac34N$ ). The ratio is $\frac{15/16}{3/4}=\frac54$ .

Topics:Radioactive decay

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JEE Main 2016 · 3 Apr · Shift 1, Physics question 22