An investment of $5000 doubles in value every decade. The function f (x) = 5000 · 2^x, where x is the number of decades, models the growth of the value of their investment. How much is the investment worth after 30 yr?

1 decade = 10 years

30 years = 3 decades

Simply plug in the value of the number of decades needed to find into where the x value is.

f (x) = 5000 · 2^x

f (3) = 5000 · 2^3 = 40000

The investment is worth $40000 after 30 years.