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:

2 N2O(g) --> 2 N2(g) + O2(g)

How many half-lives will it take for the concentration of the N20 to reach 3.13% of its original concentration?

five half lives

Explanation:

Half-life is the time required for a quantity to reduce to half of its initial value.

How many half lives it would take to reach 3.13% form 100% of it's initial concentration:

100% - 50% : First Half life

50% - 25%: Second Half life

25% - 12.5%: Third Half life

12.5% - 6.25%: Fourth Half life

6.25% - 3.125%: Fifth Half life

This means it would take five half lives to get to 3.125% (≈ 3.13%) of it's original concentration.

i believe the answer is a.