At the end of the year 2020, a banker invested $100,000 to a mutual fund, which promises a 12% return rate compounded semiannually. How many years will it take for the balance of the banker's account to double (reach $200,000)? Suppose the banker was not able to get a return rate of 12%. What is the minimum return rate (compounded semiannually) that she must receive so that the balance of her account will be at least $200,000 at the end of the year 2030 (t=10)?