a radioactive material is undergoing nuclear decay. After 40 minutes, 25% of the sample remains. What is the half-life of the sample

Answer 1

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.