need help with math problems! PLEASE HELP! i have a quiz tomorrow :(?

1.) In 2008, a car was purchased for $16,500.
Assume car values depreciate at a rate of 12% a
year.
[a] What is the decay factor?
[b] Write an equation modeling the situation.
[c] What would be the car's value this year?


decay factor is 12%, i dont kno the equation but i think its something like y=(12)x <--whats x?
and for c i got $342144000

2.) The population of a town was 12,000 in the
year 1990. After 1990, the town's population
grew at a constant rate of 8% a year.
[a] What is the growth factor?
[b] Write an equation modeling the situation.
[c] What was the population in the year 2000?
[d] In the year 2010?
[e] In 1980?

my answers were:
growth factor 5%
equation: y=12000(8)x? <--whats x?
y=12000(8)10
and got a total of 1.288490113 <--but that doesnt look right...am i doing something wrong?


3.) You conducted an experiment with a substance
with a half-life of 1 hour. Unfortunately, you
forgot to record how much of the substance you
started out with. Fortunately, you did record that
after 8 hours, there are 5g of the substance left.
How much of the substance did you start out
with?


#3 i have no idea how to do this...i dont even know where to begin...


i could really use some help this is a review + the test is tomorrow