The half life period of a radioactive substance is 5 min. The amount of substance decayed in 20 min will be

(a) 93.75%                  (b) 75%
(c) 25%                       (d) 6.25%

(a) Number of half lives in 20 min = n = 205=4 
Fraction of material remains after four half lives =116
Hence fraction that decays 1-116=1516=93.75%

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