Half lives of two radioactive substances A and B are respectively 20 minutes and 40 minutes. Initially the sample of A and B have equal number of nuclei. After 80 minutes, the ratio of remaining number of A and B nuclei is

(a) 1 : 16                   (b) 4 : 1
(c) 1 : 4                     (d) 1 : 1

(c) For 80 minutes, number of half lives of sample A=nA=8020=4 and number of half lives of sample B=nB=8040=2 Also by using N=N012n
N12nNANB=2nB2nA=2224=14

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