c) [tex]\left \{ {{2x-8y=-6\:|\times(-2)} \atop {4x-5y=-1}} \right. \iff\left \{ {{ -4x+16y=12 } \atop { 4x-5y=-1 }} \right.\\[/tex]
16y-5y = 12-1
11y = 11
y = 1
4x - 5y = -1
4x - 5 × 1 = -1
4x = 4
x = 1
d) [tex]\left \{ {{ \frac{x+y}{3}+x=15 } \atop { y-\frac{y-x}{5}=6 }} \right. \iff\left \{ {{ x+y+3x=45 } \atop { 5y-y+x=30 }} \right. \iff\left \{ {{ 4x+y=45 } \atop { 4y+x=30\:|\times(-4) }} \right. \iff\left \{ {{ 4x+y=45 } \atop { -4x-16y=-120 }} \right. \\[/tex]
y-16y = 45-120
-15y = -75
y = 5
4x + y = 45
4x + 5 = 45
4x = 40
x = 10
