Answer:
$8,000 in Treasury bills
$7,000 in Treasury bonds
$5,000 in corporate bonds
Step-by-step explanation:
Let the amount to be invested in treasury bills be $x , the amount to be invested in treasury bonds be $y
From the question, we understand that the amount she will invest in corporate bonds will be $3000 less than the amount in treasury bills;
So mathematically, this will be $(x - 3000)
So therefore, the amount invested in each will be;
x + y + x-3000 = 20,000
2x + y = 20,000 + 3000
2x + y = 23,000 ••••••••(i)
Let’s now work with the simple interests;
For treasury bills;
5% = 5/100 * x = 5x/100
For Corporate bonds ;
10% = 10/100 * (x -3000) = (x-3000)/10
For Treasury bonds 7%
7% = 7/100 * y = 7y/100
Adding all gives the total interest;
5x/100 + 7y/100 + (x-3000)/10 = 1390
Multiply through by 100
5x + 7y + 10(x-3000) = 1390 * 100
5x + 7y + 10(x-3000) = 139,000
5x + 7y + 10x -30,000 = 139,000
15x + 7y = 139,000 + 30,000
15x + 7y = 169,000 •••••••••(ii)
So we have two equations to solve simultaneously;
From i, y = 23,000 - 2x
Put this into ii
15x + 7(23,000 -2x) = 169,000
15x + 161,000 - 14x = 169,000
x + 161,000 = 169,000
x = 169,000 - 161,000
x = $8,000
y = 23,000 - 2x
y = 23,000 - 2(8,000)
= 23,000 - 16,000 = $7,000
So the last investment amount is x-3000 = 8,000 -3,000 = $5,000
