Rhonda deposited $4,227.29 into a savings account with an interest rate of 4.9% compounded monthly. about how long will it take for the account to be worth $9,000?

Since we multiply the amount of money by 1.049 (which is 1+4.9% by moving the decimal 2 spots to the right) every month, we have 4227.29 (the original rate) *(1.049)^x/1=A (amount) due to that we compound it once per month and x is the amount of months. This is a slightly edited version of A=P(1+r/n)^(nt) where n is the number of times compounded per year and t is the time in years - we simply switch years to months here.

Going back to our equation, we divide both sides by 4227.29 to get
A/4227.29= (1.049)^x. Since the end amount is 9000, we have 
9000/4227.29=(1.049)^x. Solving for x, we have
log(base1.049)(9000/4227.29)=around 15.796522523 months