let x, y, z be the amount at 6%, 9% n 12%
x+y+z = 7400
z-x-y = 200
2z = 7400+200 = 7600
z = 3800
@12%, 3800 gives 3800*0.12=456 income
0.06x+0.09y+456=744
0.06x+0.09y=288
6x+9y=28800
x+y+3800=7400
y=3600-x
6x+9(3600-x)=28800
9x-6x=9*3600-28800=3600
x=1200
y=3600-1200
=2400
