8b + 4f = 100 => 2b + f = 25 => f = 25 - 2b
18b + 11f = 235 => 18b + 11(25-2b) = 235 => 18b + 275 - 22b = 235
-4b + 275 = 235 => -4b = 235 - 275 => -4b = -40 => b = 10 kg
f = 25 - 2*10 = 25 - 20 = 5 kg
un baiat culege 10 kg si o fata culege doar 5 kg
