Edward C. answered 03/09/15
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Caltech Grad for math tutoring: Algebra through Calculus
The formula for exponential decay is y = a*bt where y is the current value, a is the initial value, b is the base (which equals 1 minus the decay rate), and t is time.
You can use the given information to figure out what b is.
20000 = 30000*b1
b = 2/3
Now that you know what b is, you can plug in 5 for t to figure out the value after five years.
y = 30000*(2/3)5 = 3950.62
So the estimated value after 5 years is $3950.62