Hi there
The formula is
A=p (1+r)^t
A future value 400
P present value 220
R interest rate 0.03
T time?
We need to solve for t
A=p (1+r)^t divide both sides by p to get
A/p=(1+r)^t take the log
Log (A/p)=t×log (1+r) divide each side by log (1+r) to get
T=log (A/p)÷log (1+r)
Now plug the numbers above
T=log(400÷220)÷log(1+0.03)
T=20.2 years round your answer to get
T=20 years
Hope it helps