How do I solve this

Question

How long will it take before twenty percent of our 1,000-gram sample of uranium-235 has decayed? [See Section 6.6 Example 13]

The decay equation is , where t is the time for the decay, and K is the characteristic of the material. Suppose T is the time it takes for half of the unstable material in a sample of a radioactive substance to decay, called its half-life. Prove that . What is K for the uranium-235? Show the steps of your reasoning.

Answer 1

0.5 = e^kt

ln 0.5 = kt

ln o.5 / t = k