(Compound value solving for n) How many years will the following take?

a. $500 to grow to $1,039.50 if invested at 5 percent compounded annually

b. $35 to grow to $53.87 if invested at 9 percent compounded annually

c. $100 to grow to $298.60 if invested at 20 percent compounded annually

d. $53 to grow to $78.76 if invested at 2 percent compounded annually