Question 1018259
He had 20,000
Invested x at 6%
Invested 20,000-x at 10%
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0.06x+0.10(20,000-x) = 1280
0.06x+2,000-0.10x = 1280
-0.04x = -720 divide, and remember -/- = +
x = 18,000 so this is at 6% and 
20,000-18,000 = 2,000 is invested at 10%
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Check:
18,000(0.06) = 1,080
2,000(0.10) =   200
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Total. . . . . . 1,280 we have the correct answer
J