daca stau 2 in (b-4) banci,3 banci goale si intr-o banca sta 1 elev
nr elevi=2*(b-4)+1
daca stau 3 in (b-11) banci , 9 banci goale si in 2 banci sta cate 1 elev
nr elevi= 3(b-11)+2
egalez si obtin
2b-8+1=3b-33+2
2b-7=3b-31
31-7=3b-2b
b=24 banci ⇒ e=2*20+1=41 elevi