P = 390,00 + 4000x
450,000 people - (15years x 4000) = 390k in 2010
15 years is the difference between 2010 and 2025
subtract 60k (that is the 15 years multiplied by 4k) from 450k and you get 390k.
Sorry if my explanations suck, I'm not great at explaining my thought process
