Question

AFLA X SI Y DACA X PLUS Y =1 000 SI X MINUS Y = 100

Answer 1

Din a doua relatie ai ca x = y + 100 ; o inlocuiesti in prima relatie =>
y + 100 + y = 1000 <=> 2y + 100 = 1000 <=> 2y = 900 <=> y = 450 => x = 450 + 100 => x = 550.

Bafta !

Answer 2

x+y=1000
x-y=100⇒  x-100=y
adunam cele 2 relatii si rezulta (x+y)+(x-y)=1000+100⇒  x+y+x-y=1100⇒
2x=1100
x=550
y=x-100=550-100=450
y=450
Verificare 
x+y=1000             550+450=1000