Hint
We usually use either $t$ or $n$ for the time variable. Will it be time in seconds, days, years, or centuries? Pick a variable for the state variable. What does the state variable represent? For a correct solution, the meaning of both these choices should be spelled out clearly.
It's probably be easiest to set up the system in difference form. The left hand side of rule would be a difference giving the change in the state variable over a given time period. Then, the right hand side gives the formula for this change.
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