!!!!!!!!!!!!!!!!!!!!!!!!!
Х+у=81
х-у=15
х+у=81
х=15+у
15+у+у=81
х=15+у
2у=66
х=15+у
у=33
х=48
X+z=6 (*)
y+z=8 (**) из (*) - (**)
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x-y+z-z =6-8
x-y = - 2
..........................................................................
(*) +(**)
x+y+2z = 6+8 (но x+y=4 )
4 +2z =14
2z = 10
______________________
x-y + 2z = -2 +10
x - y + 2z =8 <------- ответ
У^2 4×^2
--- - ----
у-2х у-2×
у^2-4×^2
--------
у-2×
C из 30 по 2=30!/28!*2!=29*30/2=29*15=435