Question

Solve by substitution
0.3x + 0.4y = 0.8
0.7x - 0.8y = -6.8

Answer 1

Explanation:

0.3x + 0.4y = 0.8 /×2

0.7x - 0.8y = -6.8

1. 0.6x + 0.8y = 1.6

2. 0.7x - 0.8y = -6.8

1+2 : 1.3x + 0y = -5.2

1.3x = -5.2

x = -5.2÷1.3

x = -4

x -> 1 : 0.6 × -4 + 0.8y = 1.6

-2.4 + 0.8y = 1.6

0.8y = 4

y = 4÷0.8

y = 5

Answer 2

x = -2

y = 10

Explanation:

(0.3 + 0.7) x + (0.4 - 0.8) y = 0.8 - 6.8

x - 0.4 y = -6 → x = 0.4 y - 6

0.3 ( 0.4 y - 6) + 0.4 y = 0.8 → 0.12y +0.4y - 1.8 = 0.8

→ 0.52 y = 2.6 → y = 2.6/0.52 = 10

x - 0.4(10) = -6 → x = -2