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The half-life period of a radioactive su...

The half-life period of a radioactive substance is `20` days. What is the time taken for `7//8th` of its original mass to disintergrate?

Text Solution

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Let the initial mass be one unit.
Mass remaining `=1 - 7/8 =1/8`
A mass of 1 unit becomes `1/2` unit in 1 half life
`1/2` unit becomes `1/4` unit in `2 ^(nd)` half line
`1/4` unit becomes `1/8` unit in `3 ^(rd)` half life
`therefore ` Time taken =3 half lifes `= 3x 20= 60` days
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