Today, Bruce and Brenda each have $300,000 in an investment account. No other contributions will be made to their investment accounts. Both have the same goal: They each want their account to reach $1.5 million, at which time each will retire. Bruce has his money invested in risk-free securities with an expected annual return of 4 percent. Brenda has her money invested in a stock fund with an expected annual return of 9 percent. How many years after Brenda retires we expect Bruce to retire? Please provide the formula, and variables used.

a) 20.01

b) 18.42

c) 22.36

d) 16.1