Use the following formula for simple interest rate:
I = P*r*t
where,
P: principal investment = 500
r: interest rate in decimal form = 3.9/100 = 0.039
t: time in years = 4
Replace the previous values into the formula for I:
I = (500)(0.039)(4)
I = 78
The interest after 4 years is $78, then, the amount of money Boden's account has after 4 years is:
$500 + $78 = $578