Question

A radioactive substance has a half life of four months. Three fourth of the substance will decay in

• A. Three months
• B. Four months
• C. Eight months
• D. Twelve months

Answer 1

The correct option is C Eight months

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Substance left udecayed, $N_0-\frac{3}{2}{N}_0=\frac{1}{4}{N}_0$

$\frac{N}{N_0}=\frac{1}{4}=(\frac{1}{2}{)}^n$

$\therefore$ Number of atoms left undecayed,

$n=2$ i.e. in two half lives

$\therefore t=nT=2\times 4=8months$

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