x = numarul total de caiete
In prima zi
x/3 + 8 = (x + 24)/3
In a doua zi
2/5 × [ x - (x + 24)/3 ]+9=
2/5 ×[ (3x - x - 24)/3]+ 9 =
2/5 × (2x - 24)/3 + 9 =
2(2x - 24)/15 + 9
In a treia zi
117 caiete
(x + 24)/3 + 2(2x - 24)/15 + 9 + 117 = x |×15
5x + 120 + 4x - 48 + 1890 = 15x
15x - 9x = 1962
6x = 1962
x = 327 carti vandute in cele 3 zile