Write a system of linear equations?

Question

A total of $15.000 is invested into two corporate bonds that pay 5% and 7% simple annual interest. the investor want to earn $880 in interest per year from the bond. how much should be invested in each bond?

Answer 1

Let x amount is invested in bond which have interest  rate 5%

Simple interest = PTR/100 ⇒ (x×1×5)/100

                         = 0.05x

The remaining amount  (15000 -x ) is invested in bond which have interest  rate 7%

Simple interest  ⇒ {(15000 - x)×1×7}/100

                         = (15000 - x)0.07

The sum of money  investor want to earn in interest per year is  $880

So  0.05x +  (15000 - x)0.07 = 880

0.05x - 0.07x +1050 =880

-0.02x = -170

0.02x = 170

x = 170/0.02

x = 8500

So to earn $880 in interest per year from the bond , investor have to invest 8500$ in bond which has 5% interest

The remaining amont 15000-8500 = 6500

Investor have to invest 6500$ in bond which has 7% interest