science
posted by Kim .
how many halflives have passed if 87.5% of a substance has decomposed? how many if 99.999% has decomposed?

Let x be the number of halflives.
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