We have a radioactive isotope called RI which has a half-life of 12 million years. A geologist has used radiometric dating and determined the rock to be approximately 36 million years old. How many half lives have taken place?

Given that,

Half-life is 12million year's

t½ = 12million years

If the element is approximately 36millions years old.

We to know how many half lives have taken place.

The first half life is

12 million years

The second half life is

2 * 12 = 24 millions years

The third half life is

3*12 = 36 million years

Since the element is 36 millions years old, so 3 half life have take place.

3

Explanation:

half life, T = 12 million years

total life, t = 36 million years

Let n be the number of half lives.

n = Total life / half life

n = 36 / 12 = 3

So, the number of half lives are 3.