Answer: 28.0 years
Explanation:
The formula we use to find the compounded amount after t years by putting a rate r on Principal value P is :

As per given , we have
P=$3200 , r=5.25%=0.0525 , A= $13400
Put all values in formula , we get

Taking log on both sides , we get

Hence, it will take 28.0 years .