# Use the exponential growth model #P(t) = P_0e^(kt)#. The half-life of thorium-229 is 7,340 years. How long will it take for a sample of this substance to decay to 20% of its original amount?

##### 1 Answer

#### Explanation:

We first find the original amt. of thorium-229, i.e., its amt.

time

Taking

We are given that, its, Half-life is

Using this data in

What we require, now, is time

By

But, from

Therefore,

Thus, it will take (approx.)

20% of its original amt.