Răspuns: 15 banci si 31 elevi
Datele problemei
Notez cu ,,b" = numarul bancilor
2 × b + 1 → cate 2 elevi in banca, ramane un elev in picioare
3 × ( b - 4 - 1 ) + 1 → cate 3 elevi in banca, raman 4 banci libere si un elev intr-o banca
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3 × ( b - 4 - 1 ) + 1 = 2 × b + 1 ⇒ elevi
3 × ( b - 5 ) + 1 = 2 × b + 1
3 × b - 15 + 1 = 2 × b + 1
3 × b - 2 × b = 1 + 14
b = 15 ( banci )
2 × 15 + 1 = 31 elevi