Let x = amount invested in short-term bonds
= amount invested in intermediate term bonds
y = amount invested in long term bonds
Then, 0.04x + 0.05x + 0.06y = 0.0495(40000)
x + x + y = 40000
0.09x + 0.06y = 1980
2x + y = 40000
From the second equation, y = 40000 - 2x
Replace y by 40000 - 2x in the first equation:
0.09x + 0.06(40000 - 2x) = 1980
0.09x + 2400 - 0.12x = 1980
-0.03x = -420
x = 14000
Invest $14000 in short term bonds, $14000 in intermediate term bonds, and $12000 in long term bonds.