Because if you take 4 percent and convert it to a decimal and multiply it by 13 it will get you the decimal that could be multiplied by 400,000 to get the amount that would be grown in 13 years. You would then have to add that amount to 400,000 to get your answer
.04 x 13 = .52
.52 x 400,00=208,000
208,000 is how many people the city has grown in 13 years but then you have to add that to the previous amount
