Discrete domains and continuous domains are both sets. However a discrete domain contains a finite number of elements and continuous can contain and infinite number of elements.
To work out how big town A will be in 1990 you need to do 200 + (200x40) because you add 40 years worth of increased population onto the starting population
To work out how big town B will be in 1990 you need to use the equation 200 X 1.1^40 because to work out plus 10 percent you need to do 1.1 and do it to the power of 40 for how many years
in 1990 town a will have a population of 8200 and town B will have a population of 9051.85