1)

[tex]15 - x = 2y \\ 4x - 3y = 27 \\ \\ x = 15 - 2y \\ 4(15 - 2y) - 3y = 27 \\ \\ 60 - 8y - 3y = 27 \\ - 11y = 27 - 60 \\ - 11y = - 33 \\ 11y = 33 \\ y = 33 \div 11 \\ y = 3 \\ \\ x = 15 - 2 \times 3 = 15 - 6 = 9[/tex]

Ответ: ( 9 ; 3 )

2)

[tex]5x - y = 6.2 \\ 0.8x + 3y = 13 \\ \\ y =5 x - 6.2 \\ 0.8x + 3(5x - 6.2) = 13 \\ 0.8x + 15x - 18.6 = 13 \\ 15.8x = 13 + 18.6 \\ 15.8x = 31.6 \\ x = 31.6 \div 15.8 \\ x = 2 \\ \\ y = 5 \times 2 - 6.2 = 10 - 6.2 = 3.8[/tex]

Ответ: ( 2 ; 3,8 )

3)

[tex]4x + 5y = 1 \\ 8x - 2y = 38 \: \: \: | \div 2 \\ \\ 4x + 5y = 1 \\ 4x - y = 19 \\ \\ 4x + 5(4x - 19) = 1 \\ y = 4x - 19 \\ \\ 4x + 20x - 95 = 1 \\ 24x = 1 + 95 \\ 24x = 96 \\ x = 96 \div 24 \\ x = 4 \\ \\ y = 4 \times 4 - 19 = 16 - 19 = - 3[/tex]

Ответ: ( 4 ; - 3 )

4)

[tex]5a - 4b = 3 \\ 2a - 3b = 11 \\ \\ a = 0.8b + 0.6\\ 2(0.8b + 0.6) - 3b = 11 \\ \\ 1.6b + 1.2 - 3b = 11 \\ - 1.4b = 11 - 1.2 \\ - 1.4b = 9.8 \\ 14b = - 98 \\ b = - 98 \div 14 \\ b = - 7 \\ \\ a = 0.8 \times ( - 7)+ 0.6 = - 5.6 + 0.6 = - 5[/tex]

Ответ: a = - 5 ; b = - 7