Solving a tax rate or interest rate problem using a system of linear equations

Question

last year Lisa had $20,000 to invest She invested some of it in an account that paid 10% simple interest per year, and she invested the rest in an account that paid 7% simple interest per year. After one year, she received a total of$1790in interest. How much did she invest in each account?

Answer 1

.1x + (20000-x).07 = 1790

.1x- .07x = 1790 - .07(20000)

.03x = 1790- 1400 = 390

x = 390/.03 = $13,000 invested at 10%

20000-x = $7,000 invested at 7%

1790/20000= 8.95% average interest rate

10-8.95 = 1.05

8.95-7 = 1.95

about twice (1.95/1.05=13/7=1300/700) as much invested at 10%