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
