HEL Calcium-41 has a half-life of 80,000 years. At the end of 240,000 years,
how much Calcium-41 would remain in a sample initially containing 60
micrograms? *
A 7.5 micrograms
B 60 micrograms
C 15 micrograms
D 30 micrograms

If the half-life is 80,000 years, then 240,000 years would be equivalent to three half-lives. In other words, the initial amount of the radioactive calcium-41 would have been reduced in half three times over. Mathematically, this can expressed as

calcium-41 remaining = initial mass of calcium-41 × (0.5)³.

Since we are given the initial mass of calcium-41, the amount of calcium-41 remaining after 240,000 years would be:

60 μg × (0.5)³ = 7.5 μg, or 7.5 micrograms.

the actual answer was 160934