COMPOUNDED
not simple interest,
compound interest is neat since you earn interest on the interest you earned
ok so what you use is F=Pe^(rt)
F=future or ending amount
P=present amount
e=base of the natural log (some digits are 2.71828182845)
r=rate in decimal
t=time in years
presen amount is 400
rate is 9% or 0.09
time is 3 years
F=400e^(0.09*3)
F=400e^(0.27)
F=523.986
round
F=523.99