If the average lifetime of a person is $72.6$ and if the person has a $0.00016\%$ chance of being killed by doing a certain activity in a year.
How do you calculate the chance of him being killed doing that activity in a lifetime?
Is it $(72.6 * 0.00016)=0.011616\% ?$ (It might sound simple but If possible, please provide a clue/explanation on how to calculate such problems.)
If the probability of a single person experiencing a certain condition (such as being killed, in your example) each year is $p$ then we can say that the probability of a single person not experiencing that condition in a year is $1-p$.
Then, the probability of a person never experiencing that condition throughout their $n$ years of life can be calculated as the product of probabilities of not experiencing that condition in every single one of those $n$ years. In your example, if a person is to not ever get killed in their life, that person should not be killed in their year 1, and not in year 2, and not in year 3, etc. So, the probability of not experiencing that certain condition in $n$ years is $(1-p)^n$ .
Now, the probability of experiencing that condition (in your example, getting shot) will be: $$1 - (1-p)^n$$
In your example, $p=\frac{405}{250.4million}=0.0000016$ and $n=72.6$ , so: $$1 - (1-p)^n = 0.000117$$ In other words, in your example a person has a nearly $0.01$ percent chance of experiencing that particular condition in their life.