Answer:
y = 900(0.91^t)
Explanation:
You want a function that describes the value of a $900 sound system that decreases in value by 9% per year.
Exponential function
When the amount of change is proportional to the amount, the amount is described by an exponential function:
y = ab^x
where 'a' is the initial value, and 'b' is the "growth factor."
Application
Here, the change is expressed as a "growth rate" of -9% per year. The growth factor is 1 added to the growth rate:
b = 1 +(-9%) = 1 -0.09 = 0.91
The initial value is given as ...
a = 900
So the function you want is ...
y = 900(0.91^t)
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Additional comment
The independent variable in this case is time, and the problem statement tells you to use 't' to represent it. The decay rate is given as a percentage per year, so the units of t will be years.