I=PRT P=present amount R=rate T=time basically 20+interest<u /> is more than or equal to 400 20+I<u>></u>400 minus 20 both sides I<u>></u>380
solve P=20 R=3%=0.03 T=? I=PRT 380<u><</u>I=PRT 380<u><</u>20(0.03)(T) 380<u><</u>0.6T divide both sides by 0.6 633.33333<u><</u>T 633.33 years at least