Half-life, t 1/2 , is the time required for a quantity to shrink to half its initial value. Radioactive elements are often defined by their half-life radioactive decay given by N(t) = N0⁻ᵗ/ᵀ is the initial quantity and τ is the element's mean lifetime.

The half-life is related to the mean lifetime by t ¹/² = t ln 2. Suppose a particular element has a half-life of 200ps. Find the mean lifetime of the decay. If there are 100gm of a particular element at t=0, how many are there after 1 ns?