notam: e nr elevilor si
b nr bancilor
e=2(b+4)
e=4(b-5), egalam cele doua relatii
4(b-5)=2(b+4)
4b-20=2b+8
4b-2b=8+20
2b=28
b=14
e=2(14+4)
e=2x18
e=36
R 36 elevi si 14 banci
verificare: 36=2x14+8 elevi care ar avea loc in 4 banci
36 elevi=9x4, si raman 5 banci libere