Answer
He invests
$\$ 32,000$ at $6\%,$
$\$ 16,000$ at $10\%.$
Work Step by Step
If he invests
x dollars at 6% interest,
y dollars at 10% interest,
then
$x=2y$ and
$0.06x+0.10y=3520\qquad\times 100$
$6x+10y=352,000$
Substituting $x=2y \ \ $into the last equation,
$6(2y)+10y=352,000$
$22y=352,000$
$y=16,000$
Back-substitute:
$x=2y=2(16,000)=32,000$
He invests
$\$ 32,000$ at $6\%,$
$\$ 16,000$ at $10\%.$
