I'm doubtful whether I've set up the equations correctly just because the computations are getting ugly:

By natural increase a city, the population of which is 40,000, will double in 50 years. There is also a net addition of 400 people per year because of people moving to and from the city.

I figger to solve for the natural increase function, where population and = years:

Now taking into consideration the additional 400 people per year,

When I compute this I get something ridiculously wrong.