12. K dollars is deposited at time t = 0, and the balance grows according to compound interest with annual effective interest rate i. Let T be the time at which the balance has reached 10K. (a) Find the exact value of T. (b) Recall the "Rule of 72." Derive a "Rule of n" that approximates T. Your estimate should be particularly accurate when i = 0.08 (c) Suppose i = 0.06. Use the results of (a) and (b) to find the exact and approximate values of T