a+b+c=90 ; a=Andra; b=Ionut; c=Crina
a+b=53
b+c=65
1.
a+(b+c)=90
a+65=90
a=90-65
a=25;
a+b=53
b=53-25
b=28
b+c=65
c=65-28
c=37
2.
a+b+c=90
a+b=53
b+c=65
53+c=90
c=90-53
c=37; b=65-37; b=28 ; a= 53-28; a=25
3.
a+b+c=90
a+b=53⇒ a= 53-b
b+c=65⇒c=65-b
53-b +b+65-b=90
118-90=b
b=28
a=53-28; a=25;
c=65-28; c=37