f + b + c = 85
f + b = 57
b + c = 46


Răspuns :

f+b+c=85
f+b=57    => 57+c=85 => c=85-57 => c= 28

b+c=46 => b+28=46 => b=46-28 => b= 18
f+b+c=85 => f+18+28=85 => f+46=85 => f=85-46 => f=39
verificare: f+b+c=85 => 39+18+28=85