If the half-life of a radioactive isotope is 32 days,how many half-lives must pass before a 3.0 gram sample is less than 0.03 g?

Question

Quizplus · Accepted Answer

We can use the formula: 
amount remaining = original amount x (1/2)^(number of half-lives)
0.03g = 3.0g x (1/2)^(number of half-lives)
0.01 = (1/2)^(number of half-lives)
Taking the logarithm of both sides: 
log(0.01) = number of half-lives x log(1/2)
number of half-lives = log(0.01) / log(1/2) = 6.6439
Rounding up to the nearest integer, we get:
number of half-lives = 7. Therefore, the answer is (C).

If the Half-Life of a Radioactive Isotope Is 32 Days,how