The radioactive isotope Bismuth-210 has a half-life of 5 days. How many days does it take for 87.5% of a given amount to decay?
A)15
B)8
C)10
D)13
E)11
F)9
G)12
H)14

Question

Quizplus · Accepted Answer

15If 87.5% of Bismuth-210 decays, then 12.5% remains. Since the half-life is 5 days, and each half-life reduces the amount by half, we can calculate the number of half-lives needed to reach 12.5% remaining. Starting with 100%, one half-life (5 days) leaves 50%, a second half-life (another 5 days) leaves 25%, and a third half-life (an additional 5 days) leaves 12.5%. Therefore, it takes 3 half-lives, or 3 x 5 = 15 days, for 87.5% to decay.

The Radioactive Isotope Bismuth-210 Has a Half-Life of 5 Days