how many half-lives have passed if 87.5% of a substance has decomposed? how many if 99.999% has decomposed?

Answer 1

Let x be the number of half-lives.

For 87.5%, 12.5% = 1/8 is left
2^-x = 1/8
x = 3

For 99.999%, 10^-5 is left
2^-x = 10^-5
x = 16.6