2х+у=6 |*2
х-2у=13
4х+2у+х-2у=12-13
5х=-1
х= -1:5
х=-0,2
Задача
пусть 1 тетрадь = х
1 карандаш = у
3x + 2y = 66
2x + 4y = 52
x = (66 - 2y)\3
2*(66 - 2y)\3 + 4y = 52
x = (66 - 2y)\3
2*(66 - 2y) = (52 - 4y)*3
x = (66 - 2y)\3
132 - 4y = 156 - 12y
x = (66 - 2y)\3
8y = 24
x = (66 - 2y)\3
y = 3
x = 20
y = 3