notam x nr biletelor de 4 RON si
y nr biletelor de 6 RON
x+y=400 (bilete) => x=400-y
4·x+ 6·y=1950 (RON) , inlocuim pe x
=> 4·(400-y)+6·y=1950
1600-4·y+6·y=1950
2·y=1950-1600
y=350:2
y=175 bilete de 6 RON
x=400-175
x=225 bilete de 4 RON
R: 225 bilete de 4 RON si 175 bilete de 6 RON
verificare: 175+225=400 bilete (A)
225·4+175·6=900+1050=1950 RON (A)