Certain attributes are for a population and are not applicable to individuals. Birth rate and death rate are two such attributes. Birth rate is the number of births taking place in a given population in a given period of time. Similarly Death rate is the number of deaths occurring in a population in a given period of time. Death rate can be calculated as Number of deaths/Total population. In the given example Number of Deaths is 3 and the total population during that period will be $18 \left(15+3\right)$ . Therefore the death rate is $\frac{3}{18}=0.166$