An investment pays 8% interest, compounded annually.

a) write an equation that expresses the amount, A, of the investment as a function of time, t, in years.

b) determine how long it will take for this investment to double in value and then to triple in value.

c) determine the percent increase in value of the account after 5 years and then after 10 years.

d) explain why the answers to parts b and c do not depend on the amount of the initial principal.

I need help with part c and d please and thank you.

for a and b my answers are:

a) A(t)=A_o(1.08)^t

b) double in value in 9 yrs and triple in value in 14 years