a + 35 = nr timbrelor primului elev
a = nr timbrelor al 2 lea elev
a + 45 =nr . Timbrelor al 3 lea elev
a +a + 35 +a + 45 = 200
3a + 80=200
3a = 200- 80
3a = 120
a =120 ÷3
a= 40
Nr timbrelor 1 elev = 40 + 35 = 75
Nr timbrelor 2 elev = 40
Nr timbrelor 3 elev = 40 + 45 = 85