Tritium is a radioactive nucleide with a half life of 4500 days, determine the time for 90% of it to decay.

We know that the number of undecayed nuclei, N, is given by N=N₀e^{-lambda t}. From this, by setting N=N₀/2 we know at this point that t= T_0.5 (the half life) and can show that lambda =ln(2)/T_0.5 . Hence we can calculate lambda as 1.54 x10^-4 days^-1. Now using the original form by setting N=0.1N₀ we can get t = -ln(0.1)/lambda=14952 days.