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Let x = how much is invested in B bonds, in dollars.
Then the amount invested at AAA bonds is 2x,
while the amount invested in A bonds is the rest, (21000-x - 2x) = (21000-3x) dollars.
Then your "investment" equation is
0.06*(2x) + 0.07*(21000-3x) + 0.10*x = 1520.
Simplify and solve for x:
0.12x + 0.07*21000 - 0.21x + 0.10x = 1520
0.01x = 1520 - 0.07*21000
0.01x = 50 ====> x = = 5000.
Answer. $5000 in B bonds; $10000 in AAA bonds; and the rest (21000-5000-10000) = 6000 dollars in A bonds.
Check. 0.06*10000 + 0.07*6000 + 0.10*5000 = 1520 dollars. ! Correct !
Solved.