a+b+c = 74
a = egrete
b = pelicani
c = berze
a+b = 42
b+c = 45
inlocuim in prima ec a+b = 42 si rezulta
42 + c = 74
c = 74 - 42
c =32 ( berze )
b+c = 45
b + 32 = 45
b = 45-32
b = 12 ( pelicani )
a+b = 42
a+12 = 42
a = 42-12
a = 30 ( egrete )
verificare : a+b+c = 74
30 + 12 + 32 = 74