1.

Thehalf life of a radioactive substance is 20s, the time taken for the sample to decay by 7//8^(th) of its initial value is

Answer»

20 s
40 s
60 s
80 s

Solution :After three half lives , the FRACTION of undecayed nuclei
`=(1/2)^3=1/8`
`therefore ` Time TAKEN for the sample to DECAY by `(1-1//8)^(th)` or `7^(th)/8` of initial value.
`=3T_(1//2)=3xx20`=60 s


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