Answer
$2000 at 5.5%
Work Step by Step
Ben will invest \$4000 at 4% and an unknown amount $x$ at 5.5%. This needs to equal the interest from investing the total amount (\$4000+x) at 4.5%:
$4000*0.04+x*0.055=(4000+x)*0.045$
We solve for $x$:
$160+x*0.055=180+x*0.045$
$x*0.01=20$
$x=2000$
Thus Ben needs to invest \$2000 at 5.5%.
