Răspuns:
Explicație pas cu pas:
a+b+c = 45
2b-c = 13
c = 9
2b = c+13 = 9+13 = 22 => b = 22:2 => b = 11
a = 45-11-9 = 45-20 => a = 25
Andrei are 25 timbre
Barbu are 11 timbre
Cosmin are 9 timbre
