# Math Help - Chemistry and Exponential Models

1. ## Chemistry and Exponential Models

Radium-221 has a half-life of 30 seconds. How long will it take for 77% of a sample to decay?

2. Originally Posted by cheer4life1694
Radium-221 has a half-life of 30 seconds. How long will it take for 77% of a sample to decay?
y = fraction of substance left after time t

h = half-life in same units as t

$y = \left(\frac{1}{2}\right)^{\frac{t}{h}}$