Question

Every ten years, the bureau of the census counts the number of people living in the united states. in 1790, the population of the u.s. was 3.93 million. by 1800, this number had grown to 5.31 million.

Answer 1

If you are trying to see how much the population is in 1800, it is  9.24 million. You add the population from 1790 and the amount it has grown for 1800.