System of linear equations

Question

last year maya had $10000 to invest. she invested some of it in an account that paid 7% simple interest per year and she invested the rest in an account that paid 5% simple interest per year. After one year, she received a total of $680 in interst. How much did she invest in each account?

Answer 1

.07x + .05(10000-x) = 680

x = amount invested at 7%

10000-x = amount invested at 5%

.07x -.05x =680-.05(10000)

.02x= 680- 500

,02x = 180

x = $9000

10000-9000= $1000

680/10000 = 68/1000 = .068 = 6.8%

7-6.8 = .2

6.8-5 =1.8

1.8/.2 = 9

there should be 9 times as much invested at 7 as at 5 percent

x/(10000-x) = 9

90000-9x = x

10x = 90000

x = 9000

100000-9000= 1000