Solution:
Let the amount invested in scheme which yields 9% be x and amount invested in scheme which yields 13% be y.
x + y = 180000 --equation 1
0.09x + 0.13y = 18000 --equation 2
Balancing the equations, multiply equation 1 with 0.09 and equation 2 with 1,
0.09x + 0.09y = 16200 -equation 3
0.09x + 0.13y = 18000 --equation4
Subtracting equation 4 from 3,
-0.04y = -1800
y = 45000
Now putting value of y in equation 1,
x + 45000 = 180000
x = 135000
The amount to be invested in scheme which yields 9% = $135,000
The amount to be invested in scheme which yields 13% = $45,000
