Cele doua carti au acelasi numar de pagini.
Notam acest numar cu n.
[tex]\displaystyle \\
\frac{n}{5} - \frac{n}{7} = 4~~~~~\text{Numitorul comun e 35.} \\ \\
\frac{7n}{35} - \frac{5n}{35} = \frac{4 \times 35}{35} \\ \\
7n-5n= 140 \\ \\
2n = 140 \\ \\
n = \frac{140}{2} = \boxed{ 70~\text{de pagini are fiecare carte.}} \\ \\
\text{Primul citeste } \frac{n}{5} = \frac{70}{5} = \boxed{14 ~\text{pagini.}} \\ \\
\text{Al doilea citeste } \frac{n}{7} = \frac{70}{7} = \boxed{10 ~\text{pagini.}} [/tex]