They may experience irritability and depression because when a person constantly smokes, his/her brain begins to rely on the chemicals it gets from the person smoking, which results in the brain "getting lazy" and not producing the chemicals anymore. If the person were to suddenly quit, the brain would lack in the chemicals it had stopped producing, making the person feel constantly annoyed and depressed because of the chemical imbalance.
If each have 12 including their selves that would be 13. 13 times 2 is 26 -2 is 24 if your including the parter then it would be 25 not including your self