A new car is purchased for $30,000 and over time its value depreciates by one half every 5 years. What is the value of the car 11 years after it was purchased, to the
nearest hundred dollars?

you could deduce the conclusion directly using the facts pertaining to the specific situation.

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answer:

at time x=0 we have 1000, at time x=1 we have 2000, at time x=2 we have 4000.  

so that's an initial value of 1000 and a ratio of 2,

y = 1000 (2^x)

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