1.
[tex]\left \{ {{x+y=20} \atop {x-y=4}} \right.\\\\ \left \{ {{x+y=20} \atop {x=4+y}} \right. \\\\4+y+y=20\\y=8\\\\x=4+8\\x=12[/tex]
x = 12
y = 8
2.
[tex]\left \{ {{x+y=5} \atop {2x+y=3}} \right.\\\\ \left \{ {{y=5-x} \atop {2x+y=3}} \right. \\\\2x+5-x=3\\x=-2\\\\y=5-(-2)\\y=7[/tex]
x = -2
y = 7
3.
[tex]\left \{ {{x-2y=-10} \atop {2x-3y=-10}} \right.\\\\ \left \{ {{x=-10+2y} \atop {2x-3y=-10}} \right. \\\\2(-10+2y)-3y=-10\\-20+4y-3y=-10\\-20+y=-10\\y=10\\\\x=-10+2*10\\x=10[/tex]
x = 10
y = 10
