system of linear equations with two variables and two equations to solve

Question

If an investor invests $24,000$24,000 into two bonds, one that pays 4%4% in simple interest, and the other paying 2%2% simple interest, and the investor earns $720$720 annual interest, how much was invested in each account?

Answer 1

.04x + .02(24-x) = .72

.04x -.02x = .72- .48

.02x = .24

x = 24/2 = $12,000 invested at 4%

24-12 = $12,000 invested at 2%

720/24000

=72/2400

= 9/300

=3/100= .03 = 3% average return

3 is half way between 2 and 4

so an equal amount will be invested at 2% as at 4%

12,000 at each, half of $24000