x+y=8
y=8-x
x+z=10
z=10-x
x+t=16
t=16-x
x+u=20
u=20-x
y+z+t+u=44
(8-x)+(10-x)+(16-x)+(20-x)=44
8-x+10-x+16-x+20-x=44
54-4x=44
54-44=4x
10=4x
x=10/4
x=2,5
x=2,5
y=8-x=8-2,5=5,5
z=10-x=10-2,5=7,5
t=16-x=16-2,5=13,5
u=20-x=20-2,5=17,5
Verificare: y+z+t+u=5,5+7,5+13,5+17,5=44