Check - roberta deposited $4,790.23 into a savings account with an interest rate of 3.8% compounded twice a year. about how long will it take for the account to be worth $9,000? a. 16 years, 9 months ? ? b. 24 years, 8 months c. 29 years, 2 months d. 33 years, 6 months

You will need to use this formula:

Years = log (total/principal) / n * log (1 + (rate / n))
where "n" is the number of compounding periods per year.

Years = log (9,000/4,790.23) / n * log (1 + (.038 / n))
Years = log ( 1.8788241901 ) / 2 * log (1.019)
Years = 0.27388614314 / 2 * 0.0081741840064
Years = 0.27388614314 / 0.0163483680128
Years = 16.7531182884
Which is about 16 years 9 months (answer is "a")

Source:
http://www.1728.org/compint3.htm

21 is the answer the answer is 21