Half-Life - A Useful Concept

On the interactive graph below, click and move the slider to the right to simulate the passage of time in a mineral. Each decay of a radioactive parent isotope leaves behind a stable daughter isotope. Notice the decrease in parent U-235 (red spots) and the simultaneous increase in daughter Pb-207 (green spots) as time passes. The red decay cruve and green growth curve represent the changing amounts of parent and daughter isotopes, respectively.
Each division on the slider represents one half-life, which is the amount of time required for one-half of a given amount of parent isotopes to decay. The half-life for U-235 is 713 million years. Other parent isotopes have shorter or longer half-lives.

 

Answer these questions, then click the "Next" Button.

1.

Approximately what percentage of parent isotopes remains after 2 half-lives have passed?
  50% 25 % 6.25% 2% 0.39%
2. If a rock initially contained 10 milligrams of a radioactive parent when it first crystallized, how much remains after 4 half-lives?
  4 mg 2.5 mg 0.625 mg .05 mg Unknown
3. What % parent remains after 10 half-lives?
  10% 1% 0.1% 0.01% Unknown
4. If a mineral contains1.56% of its original parent isotopes, how many half-lives have passed?
  6 7 8 9 1.56