3. a=nr borcanelor de 300g, b=nr borcanelor de 400g, 300g=0,3kg, 400g=0,4kg
a+b=100, b=100-a, 0.3*a+0.4*b=37, inlocuim pe b din prima relatie in cea de-a doua
0.3a+0,4(100-a)=37, 0,3a+40-0,4a=37, 0,1a=3, a=3/0,1=30 borcane de 300g
b=100-30=70 borcane de 400 g
4. [(n+52)*12]-100=1100, 12n+624-100=1100, 12n=1100+100-624=576, n=576/12=48
