Question

Substance “T” has a half-life of 4 years. 12.5% of “T” is still present in a rock sample.
How old is the rock?

Answer 1

Answer:

12 years.

Explanation:

By the definition of half life of any radioactive substance, it means that the duration in which a sample reduces to its one-half.

Now in this case, a radioacyive sample 'T' has been given with an appropriate half life of '4 years'.

Now if certain amount if 'T' sample is placed, then it is obvious that the moment it is placed, it's amount present would be '100%', as not even a second has been elapsed.

Now after 4 years, it would reduced to its one-half i.e. '50%', as its half life is '4 years'.

Now after further 4 years, i.e. a total of '8 years' it would further reduce to its one-half, i.e. the amount remaining would be '25%' as it was '50%' when we last observed.

Then after further 4 years, i.e. a total of '12 years', it would further reduce to its half, i.e. the amount now present would be half of 25% present that time, which is '12.5%'.