Use the formula: (N = N_0 left(tfrac{1}{2}right)^{t/T_{1/2}})
Rearrange to find half-life: (T_{1/2} = dfrac{t}{log_2(N_0/N)})
If the decay constant is known, use: (T_{1/2} = dfrac{ln 2}{lambda})
If the amount halves after a known time, that time is the half-life
If multiple half-lives have passed, use: remaining amount (= text{initial amount} div 2^n)
To find the number of half-lives passed, use: (n = log_2(N_0/N))
To find elapsed time, use: (t = n times T_{1/2})