23. The balance in a bank account, B dollars, is given by B = f(P, r, t) = P(1 + 0.0ir), where P dollars is the principal amount invested, r% is the annual interest rate, and t years is the time since the investment was made. (a) Find a formula for the level surface off containing the point (Pr, t) = (1000, 5, 20), and explain the significance of this surface in terms of balance. (b) Find another point on the level surface in part (a), and explain the significance of this point in terms of balance