Suppose a city has a population of 400,000 at time $t=0$, where $t$ is the number of years since 1990.
a) If the city's population decreases by 4000 people per year, find a formula for $P_1(t)$, the city's population $t$ years after 1990.

$P_1(t) =$

b) If the city's population decreases by 8 percent per year, find a formula for $P_2(t)$, the city's population $t$ years after 1990.

$P_2 (t) =$

Hint: