Răspuns :
b - numarul necunoscut
b+125.429+76.567=398.763
b+201.996=398.763
b=398.763-201.996
b=196.767
b+125.429+76.567=398.763
b+201.996=398.763
b=398.763-201.996
b=196.767
a + b + c = 398 763
125 429 + 76 567 + c = 398 763
201 996 + c = 398 763
c = 398 763 - 201 996
c = 196 767 → al treilea termen
125 429 + 76 567 + c = 398 763
201 996 + c = 398 763
c = 398 763 - 201 996
c = 196 767 → al treilea termen