You find a fossil that you think is about 350 million years old. You decide to use 235U to date a volcanic ash bed just below the key specimen. The half-life of 235U is 700 million years. If you are correct, what approximate percentage of the original 235U would you expect to remain in your sample?
a. 50 percent.
b. 100 percent .
c. 25 percent.
d. 0 percent.
e. 75 percent.

Before answer this question, you need to know what is the half-life of a compound, the term half-life is defined as the time it takes for one-half of the atoms of a radioactive material to disintegrate. Knowing this you know that has passed half of the half-time, so you should probably have 25% of the original amount of 235U. This problem may be tricky because it says half of the time to reach the half-time, but 50% of the compound will be present in 750 million years after the death of the specimen.