Question 22219
Let amount invested at 7% be "x"
Then amount invested at 9% is "10000-x"
I=Pr
EQUATION:
 7% interest + 90% interest = $840 
    0.07x + 0.09(10000-x) = 840
       7x + 90000-9x = 84000
                 -2x = -6000
                   x = 3000 (amount invested at 7%)
            10000-x = 7000 (amount invested at 9%)

Cheers,
Stan H.