Combine data from columns with the exact same variable into a new cell

Hello, I'm having a hell of a time trying to figure out how to do this and surely it is easier than I'm making it out to be, but here's my problem.

I've got a "city" in column A while column B is the "population", however, column C has broken each city into 4 different "age groups". Now column A contains every city in the U.S. including D.C. so instead of having to combine the 4 different age groups 51 times is there a formula I can wright that passes the city name as a variable in order to combine all age groups for a "population total" for each particular "city".

So, for example here's what I think needs to happen but how do I write this equation on a macrocosmic scale in order to make life so much easier:

Column B1:B4(population broken down by age group) Need to be combined to make Column D[New Column](total population per city)

Please help.

Thanks!!!