An unknown radioactive element decays into non-radioactive substances. In 720 days the radioactivity of a sample decreases by 72 percent.

Question

What is the half life of the element in days?

How long will it take 100mg to decay to 57mg

Answer 1

0.28 = e^720k

ln 0.28 = 720k

Solve for k

0.5 = e^kt

Substitute k and solve for t

57 = 100e^kt

Substitute k and solve for t