4
Correct! As you just determined, the half-life of uranium-238 is 4.5 billion years. So every 4.5 billion years, the amount of radioactive isotopes decreases by half. (After 4.5 billion years, you'd have 50% remaining; after 9 billion years, you'd have 25% remaining; after 13.5 billion years, you'd have 12.5% remaining; and so on.) Plotting these points on a graph creates a curved line, as shown here.
No. Try again.