Answer: 2.1 billion years
Explanation:
The half-life of an element refers to the amount of time it would take for half of its mass to decay.
In 700 million years, Uranium - 235 will be 1/2 of its original mass.
If divided by 2, the 1/2 becomes 1/4 which is the proportion of the original mass the Uranium would get to after another 700 million years.
Dividing by another would get the Uranium-235 to 1/8 of its original value and would take another 700 million years.
Total time taken:
= 700 + 700 + 700
= 2,100 million years
= 2.1 billion years
