First of all, the question asks us to estimate the death rate at age 45 last birthday. This gives us the age definition "last birthday" that we need to use in our calculation. So, we will need to take the total number of deaths aged 45 last birthday and divide by the total central exposed to risk at age 45 last birthday.
The total number of deaths aged 45 last birthday is the same as the total number of deaths aged 46 next birthday, which we are told is 4,800 for calendar year 2012.
For Country A, we are provided with the population using the definition "age last birthday", which is exactly what we need to calculate the exposed to risk at age 45 last birthday. That is, here we have correspondence automatically, and we can just use the population data from the table for age 45.
For Country B, we are provided with the population using the definition "age nearest birthday". This is not what we need, so we do not have correspondence. We need to adjust the population data in the table, so that it applies at "age last birthday". Since a person aged 45 last birthday must be either 45 nearest birthday (if they are in the range (45, 45½)) or 46 nearest birthday (if they are in the range (45½, 46)), we average the rows for 45 and 46 to get the required approximate populations for age 45 last birthday. (In your work, it looks like you've gone the wrong way (ie down in age, not up in age)).
Finally, it's a little unclear how you've calculated the exposed to risk using the trapezium rule. So, for each country, when calculating the exposed to risk for 2012, you will need two trapeziums to cover the whole year:
- for A, the first trapezium runs from 1/1/12 to 1/2/12 and the second from 1/2/12 to 1/1/13
- for B, the first trapezium runs from 1/1/12 to 1/8/12 and the second from 1/8/12 to 1/1/13.
The population counts on 1/1/12 and 1/1/13 are obtained by linear interpolation.