How long will it take before twenty percent of our 1,000-gram sample of uranium-235 has decayed?

The decay equation is A(t)=A0eKt , where t is the time for the decay, andKis the characteristic of the material. SupposeTis the time it takes for half of the unstable material in a sample of a radioactive substance to decay, called its half-life. Prove thatK=Tln0.5 . What isTfor the uranium-235? Show the steps of your reasoning.