Answer
The city population after 20 years is around $701,159$.
Work Step by Step
This involves exponential growth since it has a growth rate.
RECALL:
Exponential growth is represented by the formula
$y=C(1+r)^x$
where
C = initial/original amount
r = growth rate
x = number of time intervals
The given situation has:
C = 320,000
r = $4\%$ per year
x = 20 years
Substitute these values into the given formula above to have:
$y=320,000(1+4\%)^{20}
\\y=320,000(1+0.04)^{20}
\\y=320,000(1.04^{20})
\\y \approx 701,159$
