Posted by marjani on .
a radioactive material is undergoing nuclear decay. After 40 minutes, 25% of the sample remains. What is the halflife of the sample

chemistry 
DrBob222,
This is the long way but I would do this.
ln(No/N) = kt
Substitute
No = 100 (just make up any number)
N = 25 (25% of whatever you picked for No)
k = solve for this
t = 40 minutes.
Then substitute k into
k = 0.693/t_{1/2} and solve for t_{1/2}.