Chapter 5, Systems of Equations and Inequalities - Section 5.1 - Systems of Linear Equations in Two Variations - 5.1 Exercises - Page 448: 72

$\$32000$ into the first one ($6\%$), and $\$16000$ into the second one ($10\%$).

If he invests $2x$ in the first one, he invests $x$ into the second one. Then our equation is: $0.06(2x)+0.1x=3520\\0.12x+0.1x=3520\\0.22x=3520\\x=16000$ Thus, he invested $\$32000$ ($6\%$) into the first one, and $\$32000/2=\$16000$ into the second one ($10\%$).