Equation of exponential decay:

Here Y  is the final amount.

a is the initial amount.

r is the rate of change expressed as a decimal, r > 0.

t is time.

a. Write an equation to represent the loss of investment.

            (Substitute )

               (Subtract: )

b. If his investment be worth in 5 years.

                (Substitute )

(Use exponent calculator:)

            (Multiply: )

a. An equation to represent the loss of investment is .

b. If his investment be worth in 5 years is .