Question

A radioactive substance has a half-life of 5 million years. What is the age of a rock in which 25% of the original radioactive atoms remain?

Choose one answer.
a. 2.5 million years
b. 5 million years
c. 15 million years
d. 10 million years

Answer 1

d. 10 million years

Answer 2

Hence, the age of rock in which 25% of the original amount remain is:

10 million years.

Explanation:

A radioactive substance has a half-life of 5 million years.

What is the age of a rock in which 25% of the original radioactive atoms remain?

Let x be the initial amount of the substance.

Hence, 25% of the initial amount is:

x/4

i.e. in the table form the age could be represented as:

Years Amount left

5 million years x/2

10 million years x/4.

Hence, the age of rock in which 25% of the original amount remain is:

10 million years.