Answer:
18 years ( approx )
Step-by-step explanation:
Since, if an amount is compounded daily,
Then, the final amount after t years is,
A= P(1 + r/365) 365t
Where, P is the principal amount
r is the annual rate ( in decimals ),
t is the number of years,
Given,
A = $ 800,
r = 3.9% = 0.039,
P = $ 400,
By substituting the values,
800= 400 (1 + 0.039/365)365t
t = 17.774 = 18
Hence, his balance will be $800 after approximately 18 years.
7300
I CAN'T SEE IT
is the boundary of 
. We have
