Nitrous oxide (N2O) is used as an anesthetic (laughing gas) and in aerosol cans to produce whipped cream. It is also a potent greenhouse gas. The decomposition of N2O is first order. It decomposes slowly to N2 and O2:

2N2O \rightarrow 2N2 + O2

How many half-lives will it take for the concentration of the N2O to reach 2 % of its original concentration?