half lives

[boysenberry]

The half-life of radioactive elements can be used to determine the age of fossils. Uranium-235 has a half life of approximately 700 million years. If the rock surroudning a fossil was found to have 1/64th of the original amount of uranium-235, then it could be concluded that the age of the fossil is approximately:

A) 700 million years
B) 10 million years
C) 42000 million years
D) 25000 million years

How do you do this question? Do you need a calculator?  

[shinny]

1 half life=1/2 the amount
2 half lives=1/4 the amount
...etcetc
6 half lives=1/64th the amount

6 times 700 million is 42000 million years.

[boysenberry]

6 times 700 million is 42000 million years.

i guess that makes sense.