a+b+c=127
a+X=55
b+X=50
c+X=67
a+X+b+X+c+X=55+50+67
a+b+c+3X=172
127+3X=172
3X=172-127
3X=45 => X=45:3 => X=15
a=55-15 => a=40
b=50-15 => b=35
c=67-15 => c=52
solutia: X( numarul cu care adunam fiecare dintre numere)=15; a=40; b=35; c=52
verificam: 40+35+52=127